Original manga volume 5
"Boku-tachi no kizuna"

This plot is also more or less covered in the second M Ni Makasero Drama CD.
Chapter Twelve:
Yuusuke's old schoolfriend Kana is in town, and Yuusuke is dreading seeing her - as children he remembers her being someone who cried a lot, wore glasses and pretty much got in his way. He begs Arai's help to deal with her, and reluctantly Arai agrees. Little to either of them know, however, that Arai has already met Kana by accident the previous day, helping her to find her hotel after she became lost in the busy Tokyo city.

Kana, it turns out, has grown into an extremely pretty young lady, and Yuusuke realises that he doesn't want Arai's interference any further. The trouble is, Kana's developed a crush on Arai based on their first encounter and she doesn't want Yuusuke interfering!

Arai takes Kana to dinner, which is gatecrashed by Yuusuke, among other things. In the end, Kana tells Arai how she feels, threatening not to go home but to stay in Tokyo, and Arai - realising that he's puttng Yuusuke in a bad situation by continuing to play with Kana's feelings. With the help of one of his father's modelling people (who is also an old friend of his), he lies to Kana, convincing her that he and this woman are having an affair and that they are even having a baby together. Hurt by this, Kana turns to Yuusuke for reassurance, which Yuusuke gladly provides. It is Yuusuke who takes Kana to catch her train home in the end - with her telling him to "wait for me."