The commander of a variety of gacha games (Girls' Frontline, Azur Lane, Blue Oath, etc.) depicted with an S instead of his head, similar to the P-head producer in Idolmaster and the T-head admiral in Kantai Collection fanart. The S comes from the Japanese word shikikan, which means "commander". A similar example is a commander in which the face is an S as opposed to the entire head. Has a less common English language counterpart in C-head Commander.
There is another related character, the Shi-head Commander. This is like the S-head Commander, but with a し (Japanese hiragana "shi", from the reading of the first character in shikikan, 指) instead of an S. As it still starts in writing with an S (and resembles an incomplete S), it falls under this tag too.