From The TimeSplitters Wiki

Brains
Unlocked by	Challenge > Behead The Undead > Brain Drain with bronze or better
Species	Undead Animal
Size	Small
Accuracy	N/A
Agility	6
Stamina	4
Fire Proof	5
Shock Proof	5
Default AI	1
Native TimeZone	N/A
First Appearance	TSFP
Relations	Cyborg Chimp Monkey Ninja Monkey
Games	TSFP

TS1 Gesture	N/A	TSFP Gesture	Falls over and makes monkey sounds.
TS2 Gallery	N/A	TSFP Gallery	Aww... bless the little furry undead freak.

Brains is another addition to the row of monkies. This monkey is a zombie, very fast zombie, but it's a monkey. Same size as a monkey, same stats as a monkey, but with a pop off head when you get a headshot off him, therefore you can not kill him with headshot. He only appears in TimeSplitters Future Perfect.