Aqua was caught having Amatsuki as a steam friend in 2021. People found out her hololive steam account because she was friends with Marine on steam. When people found out, she privated her account and unfriended Marine because she thought that would hide it.
Over the years, people became suspicious because she and Amatsuki both tweeted they lost power at the same time once during a thunder storm. She also took a break during his live concert.
Finally, in the old "mikeneko leak" posts on 5ch from 2021, which consisted of someone claiming to be Mikeneko, she said that when she told Aqua that Mafumafu was abusing her, she laughed and said, "Oh, like Amatsuki". This was Mikeneko implying that Aqua and Amatsuki were together and he was abusing her. In one of those posts she said that "A" was in a relationship and engaged. Those posts were confirmed to have been Mikeneko for real this year when Mafumafu said he obtained the IP from those posts as part of his lawsuit and they turned out to be coming from Mikeneko's house. Whether you believe Mikeneko, a known liar is up to you.