MentorMentor is a triggered ability. “Mentor” means “Whenever this creature attacks, put a +1/+1 counter on target attacking creature with power less than this creature’s power.”
If a creature has multiple instances of mentor, each triggers separately.
An ability that triggers whenever a creature mentors another creature triggers whenever a mentor ability whose source is the first creature and whose target is the second creature resolves. (CR 702.134) (Whenever this creature attacks, put a +1/+1
counterTo counter a spell or ability is to cancel it: remove it from the stack without resolving it. A counter on an object or player is instead a marker that changes characteristics or interacts with rules or abilities; for example, a +1/+1 counter modifies a creature’s power and toughness. (CR 701.6; 122.1) on target attacking creature with lesser power.)
Creatures you control have "Whenever one or more
countersTo counter a spell or ability is to cancel it: remove it from the stack without resolving it. A counter on an object or player is instead a marker that changes characteristics or interacts with rules or abilities; for example, a +1/+1 counter modifies a creature’s power and toughness. (CR 701.6; 122.1) are put on this creature for the first time each turn, draw a card."