List your traits

This character is full of nuance. List every trait, characteristic, skill or perk that you want to incorporate to your roleplay. Separate them in categories.

There will be many!

Example:

Personality: Lonely, hunter, tactical, unsociable, manipulative.

Looks: Faceless, boneless, 4 meters head to tip of the tentacle, soft-bodied, eight-limbed, wispy yet raspy voice.

Skills: Stealth, chameleonic color change, problem-solving skills, ink throwing, infiltration.

Perks: Exotic Morphology, 9 brains, multitask, flavor detection on its tentacles.

"Onion" Strategy

There are too many traits! Playing an RPG is not like writing a book. Roleplay is all about improvisation. It is not realistic to manage that many traits at the same time.

Instead, you can group some of these traits into "Aspects". An aspect can be a small phrase or concept that gives meaning to a list of your character traits. This will make your character coherent and understandable. Instead of a big list of traits, we will have a small group of layers, like an onion, each of them coherent and easily understandable.

Example:

Elusive: Unsociable, stealth, chameleonic color change, ink throwing...

Think about forms of evading humans, social interaction, risks… as the main aspect of your character.

Curious: Octopuses are curious in real life, add this trait to elusive. Interrogatory skills, infiltration skills, scientific curiosity...

Exotic: Try adding this one too! Imagine, you do not even have a face! Reading emotions in you is hard, you express them through the colors in your skin! You do not understand human emotions, and chemicals that are toxic for humans have no effect on you and vice-versa. You can perform 8 different tasks at the same time, and you see the world through a different set of senses. It's a real challenge!!

Learning curve

The benefit of the "Onion" strategy is that every aspect is enough to play a coherent, plausible and fun character.

Don't rush. A game campaign can take years. Listen to yourself, and don't try to bite off more than you can chew.

No matter how experienced you are, this character is very alien. Give yourself time to adapt.

I would recommend you to try to roleplay no more than one aspect at a time, and add a new layer only when the first one has become your second nature.

Example:

Start with a character that is just elusive, forget everything else. Your looks will be described at the start of the session, but we will not give relevance to it yet.

One only word (elusive), that will allow you to have an idea of your course of action, how to communicate with others or what skills to use: Hiding, not sharing information, scaping conflict, etc.

When this aspect becomes your second nature (after one or two sessions), add curious to your performance. You hide, but to spy better. You do not share information with others, but you try to get it from the people you interact with. You escape frontal conflict, but to find a way around it. Elusive does NOT mean coward.

And when you are comfortable with this mixed nature, (around 4 sessions) start adding touches that reflect how exotic your character actually is. You can start with small things: describing the taste of the gun you are holding in your tentacle (Octopus tentacles can feel flavor), or how your tentacles move around endlessly getting information (each tentacle has its own brain), or how your skin turns red to try to attract a female octopus, etc. My personal approach to this would be dropping a subtle hint every hour. The point is not to give those hints too much prominence, but at the same time do not let any player forget they are sharing mission with an alien monster.

In my DM experience, it would take 4 to 6 sessions to roleplay a convincing octopus full of nuance. Don't rush it!