means A close companion or assistant, usually the secondary figure who supports a more prominent partner.

from From early-20th-century American slang, where a "side-kick" or "side-kicker" was a trusted companion. Pickpocket cant offers the likeliest root: a "kick" was the trousers pocket, and the "side kick" — the front side pocket pressed against the leg — was the hardest one for a thief to pick, hence the safest, most reliable place. From that "closest, most trustworthy" sense the word slid naturally into meaning a loyal partner who stays right at your side.