It’s a parasocial relationship thing. A reply guy is someone who responds to most (almost all) of your posts and builds an emotional attachment to you. It’s often romantic on their part and as such they will leap to your defense at the smallest slight. Sometimes they will lavish you with gifts along with their praise. It seems nice, but flip side is these guys often become jealous/obsessive and are very bad at boundaries.

Usually a reply guy is someone who mostly “replies to other posts” instead of making his own, but this is not always the case. Stay safe HP :)