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William Kempner

Marlins Career team 2026
Number #71
Position RP
Bats R
Throws R
Age 25
Height 6' 0"
Weight 222 lbs
Birth Place San Jose, CA
Born Jun 18, 2001

Key Insights

2026
The angle he throws from is unusual, and hitters notice

Kempner releases the ball from a sidearm slot, about 6 degrees above horizontal and 2.3 feet to the third-base side of the rubber.

178 tracked release points, averaging 4.5 feet off the ground.
This is the pitch he goes to when he needs the out

With two strikes Kempner goes to the sweeper: 35.2% of his two-strike pitches, against 17.7% the rest of the time.

54 two-strike pitches in 2026.
One pitch of his runs sideways more than almost anyone's

Kempner's 4-seam fastball moves 16.6 inches toward the third-base side, the biggest horizontal move in his arsenal.

86 thrown, averaging 95.3 mph.
There is one pitch of his that keeps getting hit

Contact off Kempner's 4-seam fastball leaves the bat at 93.0 mph, against 81.5 off his sweeper.

86 and 41 of them thrown.
One pitch of his spins more than almost anyone's

Kempner's sweeper spins at 2,842 rpm, the highest in his arsenal.

41 thrown in 2026.
He lives on one side of the plate

Kempner works away from right-handers: his average pitch lands 3.5 inches to that side of the middle of the plate.

178 located pitches in 2026.

Movement Profile (Induced Break)

Catcher's Perspective • Pitch movement relative to a spinless ball

Player Bio Data
DOB Jun 18, 2001
Birth Place San Jose, CA
Height 6' 0"
Weight 222 lbs LBS
Bats/Throws R/R