Samantha Travis

Samantha Travis was a child who went to high school with Queen MAB and Mediochre.

Traits
It is strongly implied that Samantha Travis had an abusive relationship of some sort with Queen MAB, who was at the time the younger and more naive of the two children. In particular, Alexander Tanner describes Travis as having manipulated and betrayed Queen MAB, and Queen MAB herself strongly implies that Travis physically harmed her in her own home at some point.

Following Mediochre's intervention, it is implied that Travis developed multiple anxiety issues, likely as a result of him having threatened her in some way.

Black, White and Shades of Mediochre
Samantha Travis is first mentioned when Mediochre is trying to convince Queen MAB to give him free reign on his investigation, and he cashes in the one remaining favour he owes her - namely "that bit in high school with Samantha Travis".

It is established that whatever the favour was, Mediochre promised he would never bring it up unless he believed he would be saving innocent lives by doing so. Bringing it up here is what convinces Queen MAB to trust him.

Caught Mediochre-Handed
Queen MAB tells Mediochre that arranging for him to adopt Aoife is a bigger favour than anything he's ever done for her, including Samantha Travis (implying that whatever he did involving Samantha Travis was the biggest favour he has ever done her).

Sent Into Mediochre-Drive
Alexander Tanner tells Joseph Carrion that Samantha Travis was an older child who manipulated and betrayed Queen MAB when they were in high school.

He refuses to give any specifics of what happened between them, but does explain that Mediochre sought Travis out when he found out what she had done.

Ostensibly, Mediochre did nothing more that speak to her. However, after their conversation, Travis's family unexpectedly left the country and Travis herself was shortly diagnosed with multiple anxiety issues. She died less than three months later, from an overdose of her prescription medication. The death was ruled a suicide, although Tanner suggests it might have been an accident. It is clear, however, that Queen MAB believes Mediochre was somehow responsible, and that it was these events which led her to consider him dangerous and create a plan to neutralise him if he became a threat to her.

Later in the book, Queen MAB also mentions that, prior to her attack by one of the fairies, Samantha Travis was the most recent person ever to lay a finger on her in her own home.