Felix's gameplay is balance rush, excelling at both offense and defense thanks to the power of his two alien friends. Much like Sheik and Zelda from the Super Smash Bros series, Cassius and Brutus can switch between each other to change modes for Felix and drastically change his normals, movement, size, specials, and supers; its like having two characters in one! Cassius is active to be an Offensive Rushdown Monster who doesn't care to defend himself (complementing his violent and overbearing nature), and Brutus being used as an Iron Wall of Defense who will always wait patiently before attempting to attack (complementing his pacifistic but self-defensive nature). But the Felix's greatest strength acts as a double-edged sword: He cannot have both modes active in the same time and its not always safe to switch between modes and excels in only one area at a time due to the time length of the transformation move, forcing him to sacrifice these two worlds for one over the other.
When Cassius is active then Felix gets an increase of movement speed, smaller hurtbox size, a hop dash, and generally easier and more damaging combo potential; at the cost of shorter ranged moves, less safe moves, and being in this form lowers his maximum health (like Vega losing his mask in Street Fighter). He also no reliable anti air or reversal in this form.
When Brutus is active then Felix takes less damage, gets safer moves, longer ranged attacks, unique armor supers, and has an easier time removing pressure off him. At the downside his attacks deal significantly less damage, gets a fatter-sized hurtbox, and not as fast movement speed. He also doesn't have the insane high/low mixups that Cassius would.
Size
Cassius Mode
Standing Hurtbox: Shorter than Average Height
Crouching and Crawling Hurtbox: Still Shorter than Average Height, about as short as Painwheel
Standing Animation looks a bit like this, only not as wide and its Cassius opening his mouth, where Felix just winds his arms:
Brutus Mode
Standing Hurtbox: Height is Average, but he's pretty damn fat and his head sticks out a bit. Felix would be susceptible to crossups in this mode because of it.
Crouching Hurtbox: Too damn Tall..
Standing Animation would look like this:
or this
(I can't decide which looks better, though Brutus is supposed to look like a bit fish-like)
Movement
Walk Speed: When Brutus is active, Felix just walks normally and has an average walk speed. When Cassius is active, he covers Felix's head in the form of a dog's face and walks on all four limbs like Painwheel would, a very fast walk that lets him go under most projectiles.
Jump: In Both Modes, Felix's jump is rather short. No Double Jump.
Dash: In Cassius Mode, Felix doesn't run or dash on the ground like everyone else, instead he hops above the ground like a frog and becomes airborne. What's interesting about it is that you can cancel the dash by pressing
to create a shorthop and you can use jump normals during the dash, which not only makes him move swiftly across the screen (if you can master shorthops), but also creates insane high/low mixups that anyone would hate to deal with. Wavedashing is completely impossible though.
In Brutus Mode, he forms a turtle shell around Felix and slides on the ground like a normal run, being invulnerable as long as it lasts. Normals can't be used during it though.
Air Dash: Cassius Mode Has no Air Dash, But Brutus does, forming a Hang glider around Felix and then flying straight forward 3/4 of the screen. Felix can only airdash forwards but the pointed front of the hang glider has a hitbox attached to it, making the dash deal damage to anyone who comes in contact with them.
Crawl: Only Cassius Mode can do it, and he forms spider legs around Felix's limbs like walking poles and crawls on the ground on all 4s, just like his normal walk. It kinda looks like Painwheel's death crawl in terms of animation, but with a smaller hurtbox and without any slashing buzzsaw.. obviously :P