Green’s Dictionary of Slang

1. (US) a hard bargainer, a cheat, a card-sharp.

1859		Bartlett Dict. Americanisms (2nd edn) 225: Keener. A very shrewd person, one sharp at a bargain.
1873		J. O’Connor Wanderings of a Vagabond 276: ‘He’s allers got the blunt, anyhow, an’ that tells he’s a keener,’ dreamily responded the first speaker.

2. (Can. juv.) a toady, a sycophant.

2001

OnLine Dict. of Playground Sl. 🌐 keener adj. boot-licker, brown-noser, suck-up, or sycophant.