From the Super Mario Wiki
Medi Guys are Shy Guys that are encountered in Shy Guy's Toy Box in Paper Mario. Goombario says they are emergency workers of the Shy Guy clan. They can restore the health of their partners in battle by four HP and ram into Mario with their flying ambulances, dealing one damage per attack. Like Sky Guys, Medi Guys are always red. However they are never seen outside battle. Also since Medi Guys are in the air, they cannot be harmmed with low ground attacks such as a hammer unless a badge is used.
Groove Guys also summon a Medi Guy sometimes.