Im looking to built a lvl 8 character that would be 100% defensive / pacifist. I was interested in a very pure, naive, almost childlike Warforged. Any advise? Possible?

To build it, sure. Available abilities don't mean you have to use them. 

Pretty sure a pacifist IRL can throw a punch - they just choose not to. Same thing. 

Would it survive in a campaign? That is up to the world/dm/campaign.

Assuming a usual campaign and you still want to be useful then look at a mage or cleric (sorcerer with devine soul is good) and focus on healing and crowd control. You will have the ethical question of "if I have someone stunned and someone else kills them am I complicit" but that is up to you to RP. 

Second option is to just do non-lethal damage constantly. Within the game rules you just have to declare it, as silly as it sounds. Problem solved.

Life domain cleric can be an effective pacifist. Just heal and buff party members while wearing heavy armor and a shield for protection.

Bard may also work for a pacifist that is an effective control caster, less armor though.

As Okahey mentioned, non-lethal damage is easy to use. It just has to be a melee weapon, not a ranged attack or spell.

You might try Wizard and just taking control and utility spells

I feel like a a true pacifist would not be living the life of an adventurer. And as Okahey stated, controlling a creature while another PC kills it is not "pacifist" either, same with buffing PCs to allow them to kill better.

 I agree with this take, the only rational reason for a 'pacifist adventurer' would be if they were somehow drafted into the adventuring life, which is very unlikely. Even being a healer wouldn't make you a pacifist, as you're simply enabling your allies to live so your enemies can die.

If you're starting at 8th level, consider a battle master. The concept of a reformed character walking a new path out of their previous path of blood is always interesting. I'm thinking Jules from Pulp Fiction ("But I'm tryin', Ringo. I'm tryin' real hard to be the shepherd.").

A good DM can always find ways for his past to catch up to them. It would be a fun ride!

You might take a look at the Oath of Redemption paladin.

A dragon born barbarian has a lot of defense at level one it has for me at least 27 hp at 20 250 something but it is a fighter so maybe it wont be a pacifist.

There are a couple of fighting styles that are good for protecting your allies with your reaction if you go fighter or paladin. Maybe ranger, I forget what fighting styles ranger gets. 

Just make sure to talk with your party about it, and consider why your character is pacifist. It's kind of hypocritical to not bloody your own hands, but consistently protect/heal/buff people that are in the process of killing, or debuff/control enemies to make them easier to kill. But that's a hypocrisy you could incorporate into RP. What would drive them to be willing to adventure and aid others in battle but not directly damage an enemy? 

Also, make sure to talk to your DM/other players about this character idea to make sure it would mesh well with the table in general. You may not want to be the one character constantly abstaining from violence and pushing for peaceful solutions in every scenario rubbing up against a more traditional party. Something to hopefully discuss at or before session 0. 

Wizard Abjurer that does not take any spell that can do damage is pretty close to what you describe. 

d4: D&D Deep Dive did an interesting battle smith build not long ago. My basic memory is they are a small race with a medium defender, mount the defender, cast sanctuary on yourself and ride around the battlefield helping your allies.

https://www.youtube.com/watch?v=CRhsxCXIuzQ&t=2383s

I have a bard that is a pacifist.  as he has spells he can take a defensive posture and serve as the group heals and support class.  I also fill in due to my college as the skills guy.  In combat situation i prefer to use spells like sleep etc.  effective in combat but not damaging.  My bard is only level 5 however so my spell choices are limited.  I'll be cross classing into sorc or peace cleric for a small dip to help bolster that.

I feel that a pacifist character would be a difficult thing to roleplay, because generally a pacifist will try to find peaceful solutions to everything and will not want the opponents to get hurt.

The workaround is to have a character who is bound by an oath to never take a life. That gives you a lot of workaround (I'll hold him while you stab him, technically I didn't kill him!) and makes for interesting roleplay. Also allows non-lethal damage.

Think about what your character will do if cornered by a wild beast. Would they consider knocking the beast out to be acceptable, or will they only try to scare it off, or just accept being eaten?

"I don't hurt people" makes you a liability in a party. "I don't kill people" makes for a much more interesting one, and one which the DM can use for revenge plots - does the party remember to tie up the people that the non-lethal fighter clobbered about the head? Or do they raise the alarm in half an hour?

You will probably have to attack some, otherwise, your character wont survive, and as people on this thread have already said, why would they want to adventure.

I think the best you will probably be able to do in 5e is to talk first, and attack after.

Quote from Jesse_DND >>

You might take a look at the Oath of Redemption paladin.

 I agree with that. Oath of redemption paladins say "Violence is a weapon of last resort." I think thats the closest you can come to a "100% defensive character."

Quote from ThorukDuckSlayer >>

I feel that a pacifist character would be a difficult thing to roleplay, because generally a pacifist will try to find peaceful solutions to everything and will not want the opponents to get hurt.

You can play a pacifist who refuses to employ violence themselves but doesn't make an issue of other people using violence- personal distaste for inflicting injury, for example.  It could potentially be a hypocritical position, but that's just a role-playing opportunity.