Although the Seven are considered the greatest superheroes of the world, some are thought to be greater than others, and thus there is a dividing line between the Big Three, and the lesser members of the group such as the Deep. Neither as lucrative, nor as powerful as Black Noir, Maeve or the Homelander, the Deep and the other members of the Seven are less of a concern to Vought American or the Boys. Furthermore the Deep has no particular connection to any of the series protagonists, so he hasn't had as much focus as the Seven's other lesser members.
The Deep's backstory and personality haven't been expanded on to any great extent, though it has been established that he was created in a similar manner to the Homelander, and he comes across as quite serious-minded and reserved. His speech is formal, and sometimes given to grandiose flourishes. He has stated that he can't remove his helmet due to an Atlantean curse, so he may be delusional.
The Deep hasn't displayed as many of the decadent or murderous tendencies of his colleagues, though this doesn't mean he's particular noble. He has shown no desire to help others, and quarrels about how much money he makes.
Powers and Abilities
The Deep can fly and has demonstrated superhuman strength and durability. While he has been seen underwater, and bears an aquatic theme it hasn't been established if he has any nautical-based powers.
In Other Media
The Boys (Amazon series)
Chace Crawford portrays the Deep. This version is Caucasian and has the first name Kevin. Unlike his comic counterpart, he cannot fly but has the ability to communicate with marine life. This is to the extent of having full conversations with them. As a result, The Deep cares deeply about aquatic life.
In the series, it's The Deep who initially forces Starlight to perform oral sex on him, implying her standing with The Seven depended on his approval. Throughout the series, he is seen harassing and making snide comments to her but eventually, she stands up to him and he ends up leaving her alone because he is secretly afraid of her powers.
Taking it even further, Starlight makes a public statement that she was sexually assaulted and The Deep is forced to make a public apology and is subsequently transferred to Sandusky, OH by Vought who didn't want to appear unsupportive of Starlight and risk their stock price decreasing.
At the end of Season 1, he is seen to have a mental breakdown over no longer being a part of The Seven.