I'm glad there is this topic as i've been mulling over this.
Hee Jae seems to obviously. I really do believe he is just a troubled kids. He looks hard but has a soft centre.
I also hope that Dong Hee isn't a suspect but he seems a bit too possessive of Chi Yeol.
Victims:
Female stalker - no one actually knows what happened to her. Is she dead? is she in hospital? has she been kidnapped and just not reported? Female stalker, aside from being a stalker hasn't really done anything wrong. Dong Hee would be the only one to know that she is being an annoyance to Chi Yeol. Or was it all at random? Can't see where Hee Jae would fit in with this one.
Young-min - Well his attitude was sure going to land him in trouble at some point. Stole Hee-yi's spot by order of his mum. Annoying Chi Yeol enough to quit. Again Dong Hee would know about this and possibly hee jae via Sun Jae. Not too sure of the motive for this one apart from him giving chi yeol some lip.
Attack on the store - Chi Yeol getting close to Haeng Seon could incite jealousy. again just seems very mindless.
So overall, i don't really know at this early stage. My true thoughts makes me think that Hee Jae has seen something but won't say. The killer/attacker could be Dong Hee who i think may also be the brother of the female student. Said brother would be around 26 years old (16 when it happened and was 10 years ago) and 'disappeared'. change of name and appearance would soon sort that out. Know one knows his origins at this time as nothing has been said much about him.
I also read above someone say that Hee-jae and Sun-jae might be the female students siblings. I do believe they are Jang hee's and Mr Lees Sons. Given that Mr Lees hasn't been on screen much might fool you for the fact they don't have the same name, but their dad is called Mr Lee.
And Geon-Hu, i don't think he is relatated at all. Think his story line with Hae-yi is just a filler.
Any other thoughts