Lumen

Lumen is...

one of the youngest of the children, and one of the most precocious. Sent to Camp Discourse by mistake (her parents thinking it was a camp for speech and debate), she fortunately proved to have a talent for witchcraft...when she's not overthinking it, that is.

Biography
Lumen was first introduced in Season 2, having arrived at Camp Discourse accidentally. This results in a running joke in which her letters to home are filled with obviously falsified information about what she's ostensibly doing at speech and debate camp.

Lumen has an extremely analytical personality, constantly trying to figure out what, if any, standardized rules magic has. Due to the nature of magic, this often leads to frustration- but, at the end of the day, she manages to pull through.

Behind the scenes
According to several interviews, Lumen was added by the writers to act as a counterbalance and voice of reason to some of the zaniness of the Eve-Nepeta-Jane trio.