So as the title explains, I'm thinking of playing a Half-Elf Shadow Monk for my next campaign. However, I want to play them a little differently this time. This time I want to make them into a ninja.

Definition? A ninja, or shinobi, was a covert agent, mercenary, or guerrilla warfare expert. Their functions would include siege and infiltration, ambush, reconnaissance, espionage, deception, and even bodyguarding and their fighting skills in martial arts, including ninjutsu.

This sounds like the Shadow Monk completely if you ask me.  But I want to give them a bit of an edge. So I was thinking of adding 3 levels of Rogue as well as taking on the Assassin subclass. Sounds like a great compliment for the Shadow Monk. Sneak attack, cunning action, and the added benefit of Assassinate.

"Starting at 3rd level, you are at your deadliest when you get the drop on your enemies. You have advantage on attack rolls against any creature that hasn't taken a turn in the combat yet. In addition, any hit you score against a creature that is surprised is a critical hit."

Because this character's goal would be to go first in combat, I was also thinking of adding the Alert feat.

Always on the lookout for danger, you gain the following benefits:

• You can't be surprised while you are conscious.
• You gain a +5 bonus to initiative.
• Other creatures don't gain advantage on attack rolls against you as a result of being unseen by you.

Now here's what I'm debating. Should I also add 2 levels of Ranger, or just fill the last 2 levels with the Shadow Monk? So I wanted to get your opinion. 3 Levels of Assassin Rogue/X Shadow Monk or 3 Levels of Assassin Rogue/2 Levels of Ranger/X Shadow Monk.

The thing is, I don’t want to spread myself too thin with my character. I've done this before playing a Bloodhunter, that had 2 levels of Fighter, and 1 level of Warlock(For RP purposes). Which is why I'm only really considering 2 Levels of Ranger and even that sounds like it might be too much.

Benefits. With these 2 levels of Ranger, my character would have Undead for their Flavored Enemy, Deft Explorer Canny, and Two-Weapon Fighting(Short Sword and a dagger).

Cons. What I stand to lose is an ability score improvement level and Opportunist,

"At 17th level, you can exploit a creature's momentary distraction when it is hit by an attack. Whenever a creature within 5 feet of you is hit by an attack made by a creature other than you, you can use your reaction to make a melee attack against that creature."

Let me know what you think.

Personally I would skip the Ranger levels and pick up the fighting style through the feat Fighting Initiate

For your description, I think the Monk Rogue path fits just fine, having two classes only usually works best. Without sacrificing ASIs after all getting +5 to your primary stat usually helps more than most feats. (There are some feats for this build which I would make high priorities, after you get 20 dex.)

Now, for added thought, thanks to Japanese Manga and Anime there are more than just the one type of Ninja, there is also the Ninja Magic user, that build I would do very differently. Maybe a Rogue Warlock or Shadow Monk Sorcerer... I would have to think it through a lot because a build like that would be M.A.D and unless you rolled up stats and got lucky I would avoid any M.A.D build. Which is why Monk is my least favorite class as it starts out very M.A.D. (Str, Dex, Wis)  you avoid some of that going Way of the Shadow. But you still have some M.A.D just taking Monk. 

As someone who played a Rogue/Shadow Monk for several months, I can confirm that's the most fun multiclass to use when trying to play as a ninja.

EDIT (this totally slipped my mind): That being said, I truly believe you're going to get a lot more out of choosing a Monk as your starting class and dipping at least 4 levels of Rogue. However, the decision is ultimately up to you, and it's not like the inverse is a terrible combination, just not as good of one. Also, if you own a copy of Xanathar's Guide to Everything, a Swashbuckler Rogue is perfect for a Monk multiclass; their Fancy Footwork and Rakish Audacity features mesh so well with a Monk.

Quote from Eldritch_Queen >>

As someone who played a Rogue/Shadow Monk for several months, I can confirm that's the most fun multiclass to use when trying to play as a ninja.

EDIT (this totally slipped my mind): That being said, I truly believe you're going to get a lot more out of choosing a Monk as your starting class and dipping at least 4 levels of Rogue. However, the decision is ultimately up to you, and it's not like the inverse is a terrible combination, just not as good of one. Also, if you own a copy of Xanathar's Guide to Everything, a Swashbuckler Rogue is perfect for a Monk multiclass; their Fancy Footwork and Rakish Audacity features mesh so well with a Monk.

IIRC, gets better proficiencies starting as a rogue. 

Eh, not really, imo. Monks don't need armor, their weapons are overshadowed by their Unarmed Strikes, I've never been in a situation where I need tools, saving throws are made moot by Diamond Soul, and the most important skills for a rogue are available to a monk. I think he'll be fine starting with Monk, but, again, it's entirely up to him.

Quote from Eldritch_Queen >>

Eh, not really, imo. Monks don't need armor, their weapons are overshadowed by their Unarmed Strikes, I've never been in a situation where I need tools, saving throws are made moot by Diamond Soul, and the most important skills for a rogue are available to a monk. I think he'll be fine starting with Monk, but, again, it's entirely up to him.

There are times when being able to use, say, a rapier, could be useful, keeping in mind you cannot sneak attack with unarmed attacks, RAW and there are no real magical monk weapons other than maybe staves. 

I mean, they both get shortswords, and you can Sneak Attack with that. It might do less damage on average than a rapier, but at least you can use something.

Mostly my reason for starting as a rogue is to get the starting equipment, so I can sell most of them. My goal was to get enough money to get the poisoner's kit.

Starting Equipment:Short Sword, Short bow and 20 Arrows, Thieves tools, Dagger, Dagger(Sell for 2 gb), Burglars pack(sell for 16), and Leather Armor(Sell for 10).

And with the background Urban Bounty Hunter, you get 20 gb.

So after selling things, that should bring me to 48 gb total.

My next plan was to convince DM I could negotiate the price of the Poisoner kit and some darts for 47.5gb(5% discount) and in exchange I'd also buy 10 darts at full price, to make it 48gb.

Plus you get more skills if you start as a rogue. More skills, less problems.

Ah, Midnight. My Tabaxi ninja. I went 4 Assassin Rogue and rest was Shadow Monk. It was a lot of fun shadow-porting to sneak attack enemies with auto-crit. 

And yes - always start Rogue. More starting skills and better starting equipment. 

Personally I'd say just stick with Monk, particularly if you're starting low level. The synergy between Monk and Rogue features is very limited since both classes bread and butter scales by class level (Sneak Attack damage and Ki points), and you'll feel that pinch hard in early play. Also, check with your DM before you plan to cash in starting gear; some DMs won't be impressed by that move and might block the attempt. And it's hard to find a good level to take the Rogue levels, imo; my rule of thumb is always to hit at least level 5 before multiclassing because there's usually a big spike in power there, and of course you'll want the Shadow teleport power from level 6 ASAP, which is then followed by Evasion and Stillness of Mind at 7th level, an ASI at 8th, and the ability to run up walls at 9th. 10th is really the earliest I'd call it where you can start getting more from Rogue than the next Monk level, and even then it's honestly a bit sketchy. You'll have a Ki pool big enough that Cunning Action is fairly superfluous by that point, and really you can get a decent skill spread with some Expertise from feats like Skill Expert (which is also a half-feat) or Prodigy since you're planning a half-elf. Plus Shadows has a strong 11th level feature, so you need to ask if putting that off is worth it. You'll already have a good spread of skill profs as a half-elf, I honestly think the feats are a better way to go skill monkey as opposed to dipping.

Be a historical shinobi, do some research into their irl tools, and embrace the truth that is Arcane Trickster Rogue.

Gaijin Goomba on Youtube has a good video on the concept of the build.

i would debate if assassin is the best subclass for your rogue to be a ninja. Sure assassinate seems plausible, but the bread and butter of a ninja was their covered operations, gathering intel. Not that much assassinating.

Thus i would propose either of these two as alternatives:

Mastermind. It gives you tools and languages and the ability to mimic speech. While also having a long range bonus action help to aid your allies, ie. distracting ninja techniques.

Scout. Nature and Survival expertise to find your targets, it plus nature to find poisonous plants for use. And the reaction to move half of your speed, as a monk, makes you hard to pin down, playing into the elusive factor of ninjas.

Personly im more of a "bland fantasy guy", make stuff as simple as posable
I would do a rouge/fighter multi class
and for the fighter i would chose the eldric knight and take some illusion spells or spells that can be used for general trickery, and any type of rouge you like
and i would get misty step or invisibility with the Fey touched or Shadow touched feat
and thats how i would make a ninja

but you do as you wishs

The problem is I wouldn’t take that many levels. And all I get for a 3rd Level Arcane Trickster is Mage Hand +2, three known spells, and two1st level slots. Doesn’t really seem worth it in the end

Yeah I’m just not feeling those 2. I mean maybe scout, but it doesn’t seem as fun as assassin.

Shinobi classs from "Adventures in Rokugan". 

I mean, for a ninja style character i had I used hexblade warlock. This does mean I used a sword, but maybe a dip in fighter could get unarmed fighting.

I thought it was lots of fun, I had shurikens (eldritch blasts), I was really fast (expeditious retreat), and had a lot of cool invocations like mask of many faces, and one with shadows to give cool kind of ninja skills. Idk shadow monk is pretty textbook ninja, but if you want a few more magic tricks up your sleeve, consider warlock.