Who is that person? (S01E18)

S01E01 You're going to be my boyfriend. S01E02 I'm not going to marry you. S01E03 You aren't going to beat me!!!! S01E04 Would you please go out with me...? S01E05 Can we do our morning study sessions together...? S01E06 I thought you live by yourself. S01E07 Is my fate going to change? S01E08 Are you going to marry him? S01E09 The Kuroshio currents beckon to me. S01E10 Don't treat me like your kid sister. S01E11 I can't forget that just happened. S01E12 It's not that I like you, yet. S01E13 Things aren't quite working out, are they? S01E14 Walls exist to be overcome!! S01E15 We have to tell her... about our secret! S01E16 I want to talk about now. S01E17 Anything could happen right now! S01E18 Who is that person? S01E19 You must have noticed by now. S01E20 I want to grow up already! S01E21 It's a secret between you and me. S01E22 I'll Be There With You S01E23 What Should We Do Now? S01E24 All That Matters Is That You Two Are Happy