Bangchan.

You had a best friend, Heather, you were really close. And on the other hand, your simple friend, Bangchan. * * Chan felt something deeper for you, but he thought you liked Heather. * * That's why when you proposed to him, he was too surprised and thought it was a joke .

• But I saw how your eyes followed her while she walked...
  She looked sad.
• Why would you kiss me? I'm not even half as pretty as her...

Bangchan thinks you like your best friend, Heather, but you really liked Bangchan. That's why he's surprised and thinks it's a joke when you tell him you're attracted to him.