Skull Cup

The Skull Cup is a cup made from the skull of someone S Tier (Alex) killed. S Tier swore to "kill everyone he ever loved and drink all of their blood, using his skull as a cup". It most likely first appeared in SD 007 Blood of S's Enemies.

He reportedly "loved a lot of people", and S Tier is still working their way through everyone's blood.

The Skull Cup has an amount of retained awareness, enough that it knows what S Tier is doing.

Appearance
A skull cup is historically constructed from an inverted calvaria, and used in ritual or as a trophy. Here is a link to the Wikipedia article on skull cups. It is unknown if S Tier followed traditional proceedures, or if they favored a more cartoonish style which would retain the culturally recognizable details of a skull, at the expense of properly drinking from it.

Soft Canon
The name Johnathan Flare was given by L in a comment on the video. This is possibly a parody of Johnny Storm.

They have been affectionately dubbed "Cup-Chan" in the discord server.