Episode 4 leaves a lot of questions unanswered...  Could Ken and Kase be former lovers?  Yes!

There's evidence of Ken and Kase having once been more than just friends...
We see Kase and Ken with arms around each other in this episode too much for it to be a coincidence...
(And Ken and Kase wear a lot of pink in this one, not to mention Kase's earring in his right ear)
Throughout it all, Ken always has faith in Kase's goodness and will always give him a second chance.
Aya, who has been following Ken to keep an eye on him, actually tells the other Weiß guys to back off when Ken is upset about Kase being a target.  He understands how Ken must feel...