Hey guys! I am currently playing a Human (Variant) Way of the Kensei Monk and just hit level 7 last session. I'm really enjoying the both the Monk class and the Subclass. But I was thinking on multiclassing him into a Level 3 hunter ranger, most likely the revised ranger (The DM's preference I think). I think it would fit his character perfectly both flavour/story-wise and mechanically speaking for multiple reasons. Mechanically, Hunters mark would be great, dueling fighting style +2 damage, +2 damage for favoured enemies (along with fighting style, sharpen the blade and Max DEX would be +10 damage, +12 damage on humanoids). Plus advantage on Initiative rolls, advantage on creatures who haven't acted on the first round (good for stunning strike procs) and mobility takes no penalty for difficult terrain (Revised ranger). In terms for his character, he is trying to hunt down a mysterious group who killed everyone at his monastery, so i thought the features that allow him to track them (Favoured enemies: Humanoids, primeval awareness) would be very fitting. My issue is I dont know when to multiclass him into it?

TL:DR - Currently level 7 Kensei monk, want to multiclass into a level 3 hunter revised ranger. When should I start into ranger levels/should I spread them out? eg) wait for monk LV 11, LV 12? etc...

It's best to compare what you would gain with what you would temporarily lose. Multiclassing into Ranger for your character's 8th level would generally be bad. You would miss out on an ASI. So personally I would at least get Monk to 8 first.

After that - do you find what you would lose in Monk for 3 levels worth what you would gain in Ranger for 3 levels? Only you can really answer that.

I mean as long as your DM knows revised ranger was never tested for Multiclassing if it fits your character then do it. I would at least go 1 more in monk before you go ranger for the ASI though.

Yeah for sure. At level 8 I can max out my Dexterity to 20. I was thinking I'd start at level 9 and go to level 11, making those 3 levels revised ranger levels. But it then pushes the "Sharpen the Blade" feature to level 14, another ASI to level 15 (I could max out Wisdom to 20 or grab a feat) and "Diamond soul" feature to level 17. I'm not sure how good this is because although I can get extra damage and RP flavour earlier, but I miss out on the cornerstone of kensei monks (Sharpen the Blade, +3 to attack and damage) until level 14. And without Diamond soul, keeping concentration (for Hunter's Mark) may prove difficult, but I have good AC for my level (AC: 21 + 2 for agile parry). Wanna see what yall think

I don't know if I would call sharpen the blade (or any one feature for that matter) the cornerstone of a kensei.  If you're using agile parry, you get a single attack per turn at +3, if you give up the AC, you can get two attacks at +3.

It very much depends on what you are hoping to gain out of a multi-class. If more damage is what you seek, one I have wanted to try with a Kensei is a multiclass with 1 or 2 levels of War Cleric. Hear me out: this is simply for the damage boost from being able to cast Divine Favor on yourself 2 to 3 times per day. Adding +1d4 radiant damage to 3-4 attacks per round is a big winner for a level 1 spell slot. If you happen to have a real nice weapon, like a Flame Blade, etc., then the extra bonus attack with a weapon becomes a fair added option when your Ki runs low, but otherwise it is not really adding much. And the +10 to hit on Channel Divinity is also fair at best, but at least you can also always use it to turn undead. But like I said, this is all about adding that extra radiant damage to each hit. And the way it is worded, it works if you use a ranged weapon attack, also, which Kensei are a little more likely to use than other subclasses.

Yeah I really considered War cleric, it has some great features for my character. I've made a lowkey promise that of our cleric dies, i'd take some cleric levels. War cleric is great, but i think that LV 3 Hunter Ranger would fit better. My DM has given me the all clear for using the revised ranger class. To clarify my thought process, here is a Pros and Cons list for the Monk LV 17 (Kensei)/Ranger LV 3 (Hunter) Multiclass (Feel free to correct me if I've missed something out):

Pros: Level 1 gives me "Favoured enemy" for +2 damage to a favoured enemy (Which will be humanoid) and Advantage on survival and intelligence checks involving them. It also gives me "Natural explorer" for advantage on initiative checks, advantage on attack rolls against enemies who havent acted yet (Which is synergizes together well and also gives me some extra stunning strike pressure). And according to the table in the players handbook (although I am not sure if this still applies to the revised ranger), I get martial weapon proficiencies as well as 1 ranger skill proficiency (Maybe stealth or investigation?). 

Level 2 gives me a fighting style, probably dueling using a longsword for +2 damage 1-handed. I also get spellcasting with 2x1st level spell slots for hunter's mark and ensnaring strike.

Level 3 Gives me the hunter subclass feature "Colossus slayer" for +1d8 damage once each round which does sound like a lot but it adds up over time and with everyone else in the party (We have a party of 7 with another who comes in and out). And I know its a bit of a longshot but on a critical hit, i can use deft strike which would make the crit 6d8 + 2d6 + 10 if everything is procked. To clarify we double our dice on a crit instead of the value and this is how my math is: 1d8 for 1 handed longsword, 1d8 for colossus slayer (if it lands on crit), and 1d8 for my martial die of deft strike, 1d6 for hunter's mark. and then double the damage die for all that, and then 2 (for dueling) + 3 (for sharpen the blade) + 5 (DEX modifier). 

Now for the Cons:

It takes a long time to set up everything: Sharpen the blade takes a bonus action, hunter's mark takes a bonus action. Setting up for max (or even decent damage) can take a while. 

Another is keeping concentration for hunter's mark: Although this is definitely annoying, my character has decent AC (21 AC + agile parry if i get it off). And I've got evasion for a little more coverage but it is still something worth considering

I miss out on Max unarmoured movement, empty body, and 3 ki points.

This is what I've worked out so far 

Colossus slayer is +1d8 damage once per round.  Empty body is advantage on all attacks against enemies that can't see invisible and damage resistance against all of their non-force attacks.

Yeah i know im missing out on empty body,  and i know that feature is super good. But it is something I wont get to level 18 (at least). I wont be using a lot compared to the 1d8 damage buff once per round. It does seem like much in a round, but throughout the entirety of battles (and heck, even the campaign) it will all stack up. especially considering our party's build. My AC and HP is decent already at level 7, the ranger features as a whole would come online earlier, and I don't even know if my campaign will go to level 18. But empty body is very tempting, you're right. I also feel that the way my DM sets up boss fights, at level 18 he will definitely have enemies with true sight or ways of detecting invisible entities. 

Do you think I should take 2 levels in something instead of 3 levels?

Probably the most common two-level dips are Warlock for Invocations or Fighter for a Fighting Style and Action Surge.

Doubt they will give as much as Revised Ranger. They’re very front loaded.

I can't multiclass into warlock cause my charisma is very low (8). And I was thinking about a fighter multiclass dip, but action surge is once per short rest. While Revised ranger gives me soooooo much more. Also, it suits my character flavour-wise/suits backstory