Of all the characters that Stephen interacts with in Portrait, Cranly is the most interesting. He also ends up having an importance influence on Stephen towards the end of the novel.
The first time we meet Cranly (other than the roll-call in the classroom), he is almost ironically acting the comic character, questioning and answering Stephen in Latin. Cranly never seemed to be the bubbling, bouncy, smiling character even when he had his funny moments (and is described as "brooding", "sour", "bitter", "watchful", etc. to give us this impression). Later we see that Cranly acts the complete opposite--when he overemphasizes his annoyance by shouting/screaming/shoving (Temple on multiple occasions and then the portly kid who farts on the steps). I don't think Cranly is actually that angry (his "victims" always seem to be in good humor, maybe he acts like that a lot and they know he isn't really all that angry). Which again, makes him an almost comical character.
The main impression I got from Cranly is that he was the most mature student at the college. He sort of has this attitude like "Cmon guys, seriously?" over a lot of things that he finds petty. For example when Maccan and Stephen are sparring over the Tsar's peace petition paper, Cranly is sort of like "Are you guys serious? Can't get go play handball? etc.". Later when Cranly and Stephen are talking, Cranly acts as the voice of maturity and reason when he calls Stephen out on his babyish behavior regarding Stephen serving in church service. Stephen doesn't want to out of principle or whatever, and Cranly again has that "C'mon, seriously?" vibe going on. "Just do it dude, it's a small act... it wont kill you, it will make your mom happy" etc.
In class it was briefly mentioned that Cranly served the purpose of trying to bring out a little humanity in Stephen, and I think that adding this element to his character really gives him a lot more depth.
"Have you ever loved anyone?"
"I ask if you even felt love towards anyone or anything".
"Your mother must have gone through a good deal of suffering. Would you not try to save her from suffering more even if... or would you?"
"Do as she wishes you to do. What is it for you?"
Cranley asks Stephen.
These sort of questions only act to reaffirm Cranly's maturity.