Arthur Maxson

 is the last descendant of the Maxson dynasty, the founders and traditional leaders of the Brotherhood of Steel. He was born in 2267 to Jonathan and Jessica who are both deceased by 2277. Still a child and ranked as Squire, he has been placed under the supervision of Elder Owyn Lyons. Due to internal conflicts in the Brotherhood of Steel this move was thought to increase his personal safety; he is considered to be safer in the Citadel than in the Brotherhood's homelands, even though it is located in enemy territory.

He can usually be found in the Citadel's B-Ring where his room (and personal terminal) is located.

One of his dialogue lines is what links the FO3 Enclave Power Armor to previous Fallout games' "Advanced Power Armor Mk II". He is also the only character, besides Star Paladin Cross, who recites lines from the Codex.

From his computer (which you can hack without Karma penalties), you can find that he writes stories, has a crush on Sarah Lyons, and that there is a Guns and Bullets magazine underneath a bed in the den.

He also wears only one of three Junior Officer Outfits, which can be obtained by Reverse-Pickpocketing. The outfit provides no benefits other than an amusing look on an adult character.

Appearances
Squire  appears only in Fallout 3.

Squill -- This is his brother, if you pass the speech challenge, there will be an option for him to lick your bellybutton. Doing so will give you 40-100 HP, depending on your level.

Behind the scenes
 is likely a recreation of Arthur Pendragon (King Arthur). He is the rightful heir to the Brotherhood of Steel, but has been sent to live with a foster father for his safety. His Kingdom is divided and the factions cannot agree upon a leader. Arthur Maxson is being trained in the ideal of might for right, which his Kingdom has never followed.