[Daigoro]

HK movies are full of these kinds of soap opera style characters.

Other characters could be-
- an ex-cop who's joined the triad, who might get info about an undercover cop from his old police contacts

- another undercover cop from a different division, or maybe from the mainland police

- the triad son of the undercover cop's boss, so there's a risk of them crossing paths at a social function

- a triad guy who's been tipped off about an undercover somewhere in the organisation, and who might get clues later that he's actually in the party

- a triad guy who knows the identity of the undercover cop, but is honour-bound from betraying him because he saved his life, gave him money for his sick wife, adopted his abandoned bastard son, or was a close childhood friend

Those are all-linked to the undercover PC somehow, which actually fits the HK style of plot-writing, but to get away from that, you could have-

- the son of the boss, who's just around to have fun but hasn't learnt the true meaning of loyalty or honour yet

- a hot blooded young gangster, who competes with the undercover PC to be the fastest to rise through the triad ranks. This causes tension for the undercover PC, who has to stay in competition with him yet avoid going over the top in killing innocents, shooting at cops or causing mayhem

- a stodgy, older triad guy who is still low rank, who is reliable and trustworthy and has survived for so long due to a lack of ambition. Sounds a bit dull as a PC, but a common trope in HK films, and he could be hiding some particularly juicy secret of his own, such as that he's the real boss of the triad, or even the elusive triad grandfather that the three triads mutually pay respect to