Li'l Max
From Inkipedia, the Splatoon wiki
Li'l Max is a sticker that represents the max rank obtainable in Splatoon 2. It is also a Mem cake and can be obtained in Ink It Station.
Appearance
Li'l Max looks like a stylised version of the Rank S+99. When Rank X was introduced, a new version was released that resembles Rank X instead.