Sentinel Unit 23

 is a fictional robot character featured in comic book titles published by Marvel Comics. It is considered part of the mainstream Marvel Universe, colloquially referred to as the Earth-616 reality. It first appeared in Avengers, Volume 1 #103 in September, 1972 in a story titled, "The Sentinels are Alive and Well!".

Overview
is a Mark III series Sentinel. It is one of the units created at the Sentinel manufacturing facility known as the Ant Hill, located in the Australian Outback.

Biography
was one of the Sentinels that operated under Sentinel Unit 2 at the Ant Hill. The Sentinels engaged in a plot to instigate a solar flare, which would sterilize Earth's mutant population, thus bringing the race one step closer to extinction. The Sentinels fought up against the Avengers and were defeated. was disabled and left behind in the Ant Hill, which was sealed up by Iron Man.

Years later, a mutated Sentinel from another timeline, Sentinel 3.14159, discovered the Ant Hill and re-activated the existing Sentinels, including.