@zz If you think this manga is just about revenge, the you are not seeing the full picture... Aqua's character arc most likely is about him realising he can not center his life around revenge, that there are other things more valuable than clinging to the past or to hatred... He needs to find the culprit to find closure, but at the same time, his life has to open up to more positive feelings and treasuring the relationships he has with people who are alive... Therefore, him falling in love could help the narrative... Of course, it doesn't have to be romantic love, the character development could focus on friendship or even his relationship with his sister.