Yuyu's personality from when Riri first met two years ago her seems a lot more happy than the Yuyu she met. Considering Yuyu had a partner in the flashbacks while in the present day she works alone, something bad definitely happened between then, which we get to see in the third episode.
Once Yuyu and Riri get a Schutzengel/Schild pledge at the end of the second episode, Yuyu is shown discussing with her roommate (who is later confirmed to be dead and possibly a figment of her imagination), saying she hopes she regrets the decision.
We get to see a bit of Yuyu's secret ... she goes into an Unstoppable Rage mode trying to kill a Huge, and she's immediately horrified that Riri has to witness this truth. It's also confirmed that this ability accidentally killed her previous partner.
The sixth episode makes Yuyu trigger her Unstoppable Rage mode yet again once she sees her old CHARM inside of a Huge. She's led to believe that Riri was killed as well, making her even attack Riri as well (while Riri had to reassure her that the others saved her).
The ninth episode has a ton for several characters.
The ninth episode is one big Trauma Conga Line for Riri. As shown on the caption image, she was happily cutting Yuri's hair in the morning. She was then approached by the Student Council, ordering her to give Yuri away. Yuyu stealthily ordered Riri to hide, which made the government put an arrest warrant on Riri. Yuri herself was able to sense sadness on Riri, despite the fact Riri was trying her best to hide it. After the government rescinded the arrest order thanks to Moyu, a Huge suddenly attacked, letting Yuri go and destroy the Huge at the cost of her own life. You can see Riri trying to keep her composure, but she's clearly crying.
Kaede expressing disappointment in her father for the failure of the G.E.H.E.N.A. experiment, even wanting to disown him as well.
Tazusa reveals that she was experimented by G.E.H.E.N.A. - she mentions that they messed with her body and she only made it out when Yurigaoka took her in. You can even see her clench her fists as she says this too. This is also considering she's very untrustworthy towards most humans.
