$80:825E

From ZAMN Hacking
ROM address data
Length 0x80
Type Code
Name createEntity
Description Creates an entity
Arguments/Return value Y+A = [pointer32] Entity pointer
Returns A = [uint16] Entity ID
Return type RTL

Note: This will return 0 if there are no empty entity slots available. 0 is technically a valid entity ID, but this ID will always be used for the first player entity (see $80:8248).