Air carrier First Officer reported the Captain descended below charted altitude on a visual approach to STL.

Narrative

Flying the KAYLA 3 Arrival for Runway 11 at STL the Mode Control Panel (MCP) was LNAV/VNAV and the Multifunction Flight Display (MFD) displaying LNAV/VNAV path. While heading to HIXOB on the ILS we were cleared for the visual at about 18 miles from the runway. Pilot flying (PF) selected JEXAR and set altitude of 2500 ft.; once pilot monitoring (PM) verified it was executed. Intercepting final at JEXAR and on path zeros were set to fly as VNAV overlay and then switch ILS as previously briefed.At approximately 800 ft. AGL PM called out altitude low. Almost simultaneously the PF was disconnecting Autopilot and throttles; adding thrust and leveled the aircraft. As this happened 'Below G/S' annunciator flashed momentarily. PF flying hand flew aircraft to intercept PAPI lights to safe landing on Runway 11. On a side note; during the roll out the left thrust reverser lever wouldn't go past the electromechanical lock preventing it from deploying. PM made callout further confirming only right reverser deployed. After a slight delay the left reverser deployed. With over 6000 ft. of runway remaining the was no risk of runoff.