The pair exchanged thousands of texts and messages which a disciplinary tribunal heard were "increasingly intimate and intense".
Mepham told the inquest that she didn't know how often Back had been texting and emailing the teenager.
"I'm not upset at his choices. He's a good man, a very good man and only does things for the goodness of others."
One text to Reiha read "I so badly want to climb in your window right now, just jump through and tell you how glad I am that you are here".
The relationship also involved overnight stays at the couple's house.
Something Mepham says she argued with Back about.
"He gave his reasons and I guess I let it happen."
The first night Reiha came over, Mepham told the inquest it had been raining heavily.
"She was so upset and drenched. I was worried she was becoming hypothermic."
The family produced evidence that on that day there was minimal rainfall.
"Possibly it was raining, possibly it was not that day," Mepham told the inquest.
Later she thought that it could have been several days later.
Mepham claimed it was hard to say no to the 13-year-old's request to stay.
"Reiha had a way about her that it was quite difficult when she talked you through things. In some ways, and this is no excuse, you felt you needed to do what she asked."
Reiha stayed that night and up to six nights more. Twice when Mepham wasn't there.
On one occasion she and Back met Reiha in their car. Police discovered the three of them parked up. When they went to take her back to her parents Mepham says Reiha "freaked out".
"She was digging her fingernails into my arms, hyperventilating in the footwell of the car. She was the most inconsolable I have ever seen a human being."
When asked why the teenager might have been upset at home, Mepham said Reiha talked of her isolation because her older siblings had left home and that she had trouble getting on with her mum.
"It was not something devastating or frightening. She was left alone a lot and needed emotional support that she wasn't getting."
Mepham agreed that Reiha had become emotionally dependent on Back. Of the relationship she says they were "obviously close".
She described the energy between them as "overpowering".
"They were both big energy people."
Mepham told the inquest she had "never tried to undermine" Reiha's actual family.
Called on to explain why she had kept their sleepovers a secret from Reiha's parents and the school, Mepham said it was "possibly, partly" because she would be in trouble with the teachers' council.
She denied there was "a code of silence" between them.
When asked why she didn't tell police, Mepham admitted that was a mistake.
Barrister for the family Moira Macnab suggested Mepham made a choice to lie.
"I did not tell the whole truth, no. I wish I had spoken to police about what I knew."
Following Reiha's death, Angela Mepham and her partner, Sam Back, were found guilty of serious misconduct by the Education Council's Disciplinary Tribunal.
Back was deregistered, Mepham censured. No criminal charges were laid.
The inquest will resume again in mid December.
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