A depiction of Blue Archive's Sensei (the player character), originating from Arona's drawing of the sensei. He is presented as a child's drawing caricature of a man with sparse hair and a rather distinctive "smile".
He is frequently used to represent the perverse side of the fanbase, and by far the most common depiction of sensei within the Blue Archive community.
Typically, Doodle sensei is depicted literally as a doodle from head to toe. Though he is also often depicted (particularly in NSFW works) as a faceless male with a piece of paper depicting the doodle attached on his face as a mask. In rare cases, the entire head will be replaced with the doodle's version instead.
This tag implicates sensei_(blue_archive) (learn more).