I usually just lurk in these threads, but as someone who sympathizes with Yuu, I think I should say my piece lol. This story is going on a hiatus so probably a good enough time. Im probably one of,if ANY who find him really relatable because of similar circumstances. So his actions make perfect sense to me.
Yuu's defining character trait is his INSECURITY issues. I say this as someone with kinda similar personal experience. Its not him having low emotional intelligence like readers are accusing him of. Its not that he doesnt think about anyone else. We can clearly see in the novel that he did notice Hikari was acting strange, also that Aya was hiding something during their relationship. No, the difference comes from the fact that he didnt fully believe that Aya even liked him romantically.
Think about it, Yuu and Yami basically met on roadside, as complete strangers. Yami literally only wanted to bone him because she was in a bad mental state as an escape for her, even Yuu saw through this. Then during when they first tried to have sex, she did not even look him in the eyes. She was like she wasnt even doing it wholeheartedly. Even when she gets the physical aspect properly crossed, shes still avoidant and distant when it was about revealing anything sensitive or personal. She even pushed back on his attempts to get closer. Keep in mind this was after Yuu utterly failed at getting with Hikari (in his mind). Putting myself in his shoes, I would 100% feel that Aya was only looking for an emotional escape and not committed relationship. This feeling got lesser as time progressed and he started falling in love with her, but somewhere at the back of his mind he constantly felt uneasy and thought she was going to leave him because he is not worth it. Then one fine day she suddenly disappears from his life and blocks all his attempts to contact her. Obviously he would come to the conclusion that she just got tired of him. For him,going to search her doesnt make sense. She dumped him for a reason. Even I wouldn't ,its not about what I feel but respecting what she feels. He likely also pinned the reason for Aya leaving him being that he still had feelings for Hikari. Which explains him not getting into any relationship after Aya's imploded.
THIS IS THE REASON why he so easily accepted Aya's words of her getting tired of him. That is something he had been believing all along, but the encounter only sealed it for him. It was not because he didnt care about Aya. It was not because he didnt care about her that he accepted her family situation he just didnt feel it was his place to butt in as someone who Aya didnt see worthy as. That is also why he wanted to stay friends with Aya, he wasnt hoping for a second fallback option for him, he just saw it as natural because Aya didnt see him as romantic partner anymore but she didnt say she hated him. There is no second thoughts there he is wearing his heart on sleeve.
Similarly him not telling Hikari about Aya is merely because of his past experiences. He already has supposed experience of his ex dumping him what he believes because of his past hangover. Not only that he finds out his ex and crush are even best friends. Idk about u guys but I would 100% think telling Hikari is a death sentence if I was there, because I would also have had to tell her that Aya dumped me because she got tired of me. That is the BIGGEST red flag for any girl, coming directly from your best friend. Hikari would also come to know that (according to Yuu) him not forgetting Hikari caused the relationship to fall off, so Hikari would also think (according to Yuu) that he would never forget Aya. Yuu likely completely blames himself for the failure of Aya's relationship. Worst case scenario, Hikari may become estranged from him as a whole because she sided with Aya completely.
I find it shocking readers here think indecisiveness defines his character, that is not at all the read I got from him. He has never run away from making a choice. He just doesnt think he is someone worth loving so he greatly underestimates the feelings both girls have for him. But the boy doesnt have eyes for both girls.
P.S. I also saw comments saying he never tried to get with Hikari, we dont know if he did or not, we know he had 3 heartbreaks with her so the first heartbreak likely was him trying from his side indirectly but not being reciprocated
