The fact that the last thing we hear from Mato as she fuses with BRS in episode 5 is her distorted scream.
It may be not so much but Yomi's Creator Breakdown painting of Mato can be considered as one. See Episode 5.
Again in the same episode. Yuu is grateful to receive a bracelet from Yomi, only to find out later that it has some of her hair.
Insane Black★Rock Shooter's beatdown on Mato!Black★Rock Shooter, due to the fact that Mato isn't used to the amount of pain Otherselves go through, and spends most of the fight not wanting to harm IB★RS, while the other is none-too-reluctant to do so.