Though Thanatos is assumed to be a relative of the Great White shark based upon his appearance, his species is unknown, as he is far larger than any other Great White (and any other kind of shark that appears in the game, on that note). As well as this, he has a darker colouration than the normal grey of a Great White, though his belly is still pale. The places on his body where the dark upper-body colour blends into the pale belly colour are uneven, almost spotted or patchy-looking, and his fins appear to be larger than the proportions of a Great White would normally allow, as well as being somewhat ragged-looking. He is covered with scars from countless battles, as he often attacks boats and other creatures (thus provoking a response in the form of a counterattack, wounding the creature).
This shark is a vicious creature, feared throughout the Aegean Sea in his resident video game for his aggressive and violent nature. This is reflected in his tendency to go out of his way to attack the player character, as is seen multiple times in the main story and even when encountered in his natural territory. It is reported that he is extremely cunning, evading capture after years of attempts upon his life and freedom. He is said to be seen in waters all over the world, but only rarely, and is feared by all who know of him.
It is said by Hayako Sakurai that Thanatos seems to have a particular taste for the player character, even noting that "it seems like he's got a liking on you". This is strange, considering that Thanatos's viciousness usually seems to be universal, but it is likely just an explanation for the shark's A.I. only being programmed to target the player character and not other divers.
Thanatos shakes off the effects of the Pulsar (a healing device that also calms aggressive animals down in-game) very quickly, making it very difficult to get close enough to focus on him in order to obtain his in-game information (for cataloguing uses) or to take a picture of him. He is one of two creatures that does this, the other being a deformed Goblin-Shark-like creature called Okeanos's Guardian.
- Thanatos is also the name for the Greek god of Death (not to be confused with Hades, the God of the Underworld). This reflects his love for destruction, and also makes Thanatos a very popular name for villains in video games and media.
- Thanatos has a following of normal Great White sharks that follow him around and attack the player character as well. It is unknown whether they are following some kind of order given by Thanatos or doing this of their own volition, but even though Great Whites are renowned as being vicious (Thanks in large part to the movie Jaws), sharks are not usually an aggressive species by nature, making it more likely that Thanatos is somehow influencing them.