This was a good chapter. Nice fight and an unexpected twist. I was anticipating Sting finally showing up though considering Rakheid's magic also encompasses substantial light magic.
That's based on the assumption she is actually aware of Rakheid's existence. When she was being held prisoner, she saw Rakheid and was wondering about his magic which made her feel uneasy. If she knew that Rakheid is supposed to be her son, her reaction would have been quite differently.
Well Mavis simply doesn't seem to know about Rakheid. Either she has amnesia, didn't know she gave birth at one point or Rakheid was conceived and born in an unconventional way, but anyway you can't mention a child you didn't know you had. And in the case of Zeref, well why would he? In fact most likely at least all of the Spriggans know about it, but for them there is no particular reason to talk about it all the time. Until recently the Ishgar continent was of little to no importance to them, so for them it's rather mundane information.
She was 13 on the moment she stopped growing due to the curse, which makes her one year older than Wendy at the moment when the latter was introduced in the series.
