Ally who's not completely an ally

• Faux ally
• Supporter to keep a watchful eye on
• Reggie, to Archie, in Archie Comics
• Buddy to be wary of
• Fake chum
• Bud that you're not always high on?
• Semi-opponent
• Rival who's somewhat on your side

Wiktionary Word definitions in Wiktionary
n. 1 (context humorous English) Someone who pretends to be your friend, but is really your enemy. 2 (context humorous English) A fair-weather friend who is also a rival.