Left to right: Proteus, Triton

Proteus and Triton are the two children of Poseidon. Both are male.

As Poseidon was having difficulty conceiving a biological child with his wife, they decided to adopt Triton, though they kept on trying. Much later on, Proteus was laid and hatched as an actual biological son.

Proteus is merely a baby at this point, so he doesn't really have a unique personality yet at this point.

Triton is an adopted son, but he doesn't know about it yet. Despite this, he loves his parents dearly, and they treat him as if he's one of their own. Triton is surprisingly mature and independent for his age, as he was the only child for quite some time.
Triton has a long-lost biological brother with whom he lost contact with ever since he was adopted.

Nevrean species by  Mick39

BTW I am NOT gonna make all 16 moons of Neptune!