Agent 8

INK: As an Inkfish, Eight's body produces viscous, brightly-colored ink that he can drain into a tank and use as a projectile. Eight can paint this ink onto certain surfaces and rapidly swim through it in his octopus form. It will only stick to textured surfaces like concrete, stucco, or rough wood. Smooth surfaces like plastic, vinyl or lacquered wood won't hold it, the ink simply sliding off into a puddle on the floor. The ink is very sticky and difficult to walk through, except to fellow Inkfish who are using the same color ink. Eight regenerates ink quickly, filling up a tank from empty in approximately 10 seconds while in humanoid form and about 5 submerged in ink as he can reabsorb the ink he's already put down. The ink will remain for about 4-5 minutes after exposure to air before breaking down and dissolving. The ink is not poisonous and is perfectly safe for consumption.
SWIM FORM: Eight can change from humanoid to octopus at will, with different abilities in each form. His humanoid form grants him dexterity, mobility on dry land, and the ability to use weapons, When in swim form he can rapidly swim through his ink for quick escapes or stealthy approaches, however he is completely defenseless in this form and his tentacles are exclusively for swimming and are too short to allow for effective climbing like a real octopus. In swim form, he can also fit into tight spaces as by collapsing the bone-like organs that give his humanoid form structure. The transformation is practically instantaneous and takes no energy. In canon this allows him to phase through wire mesh with no problem. That feels game-breaking to me, so the compromise I propose is that this works like real-world octopus: He can only get into the spaces he can fit his beak through. His beak is roughly the size and shape of a set of human teeth, but is one solid mass instead of individual bones. This ability does have a pretty big downside: it requires his skin to be so thin that if he falls into water,the fluids in his body will diffuse and he will straight-up dissolve.
SUPER JUMP: While in Swim Form, Eight can pressurize the ink in his body and expel it, sending him rocketing through the air at high speeds. Without any enhancements, Eight can send himself about the length of a football field and reach the height of the goalposts. He cannot change trajectory or distance while in mid-air and needs solid ground to have forward momentum.
FALL DURABILITY: He can fall from heights without sustaining too much damage. The canon explanation is that he doesn't have bones and therefore can't break them. However, we have seen other Inkfish get things like sprains before, so I'm going with that while he can't break bones, bad falls can still cause sprains, pulled muscles, bruises, etc. Any fall that would kill a human would also kill Eight.
CHROMATOPHORES: Eight can change the color of his ink, tentacles, skin, and eyes at will. It has very limited use for camouflage because he can't incorporate textures, just change into solid colors. The fanciest he can go is give the tips of his fingers and tentacles a gradient into a second color.
TENTACLE BRAINS: I can't imagine when it will ever come up, unless someone has a really good idea on how to incorporate it into a plot, but I'm including this for sake of completion. Like a real life octopus, Eight has rudimentary brains in some of his tentacles. All this is doing for him is keeping his "hairstyle" in place without him needing to consciously remind his "short-hairs" to stay put. However it IS possible to sever those tentacles, hook them up to machines, and have them function as a simple intelligence. His people do this to run most of their weaponry as the tentacle brains can recognize threats and run simple attack routines.
 

IN CHARACTER

PHYSICAL AFFECTION: Eight isn't one to initiate but he'll respond well. Note that Eight isn't a mammal, and feels cool rather than warm to the touch.
PHYSICAL VIOLENCE: Eight hates violence and won't initiate it if he can at all help it...but he can absolutely dish it out. The fact that he fights with ink over violent weapons doesn't even slow him down. If he can't splat you, he'll figure something else out.
RELATIONSHIPS: Eight is often very shy on a first meeting, but has an open heart and accepting of people and makes friends quickly. He's never been in a romantic relationship and might not recognize or trust if he's being flirted with at first.
PSYCHIC & PSIONIC INFORMATION: His memories from the first 14 years of his life are almost completely missing, as they were physically removed from his mind. These memories cannot be restored or otherwise accessed. He has been told of his own past so his mind will fill in the gaps but he can't remember anything concrete. We can discuss how that looks to your character.
MEDICAL INFORMATION: He has three hearts. He has no bones. His blood is ink. His skin will dissolve if he's submerged in water. His biology is kind of a mess all around.
MAGICAL INFORMATION: No magical abilities
OLIFACTORY: Eight keeps himself quite clean, but sharp nosed characters will be able to smell ink and a small hint of fresh fish. 
OFFENSIVE SUBJECTS & TRIGGERS: Eight is a wartime refugee expat living as a minority in his city. He's sensitive to discrimination and prejudice and especially to dehumanization .

OUT OF CHARACTER

 

BACKTAGGING: Absolutely!
THREADHOPPING: Sure!
FOURTHWALLING: No
NOT INTERESTED IN: Smut, mostly