Kindred is the real Harry Osborn
Specifically, he's the original, pre-One More Day Harry; the one who became the Green Goblin and died. The Harry Osborn who came back from Europe, married Liz and had kids was just a clone created by Mephisto with all of Harry's memories, right up until the storyline where Harry died; after the deal to erase Peter and Mary Jane's marriage, he didn't re-write history, just altered everyone but MJ's memories, including thinking that Harry never died. The real Harry was brought down to Hell and transformed into Kindred, explaining his grudge against Peter. Alternatively, Kindred is The Stacy Twins, Gabriel and Sarah
In recent issues of Amazing Spider-Man, a sin-free Norman Osborn is left a key by his deceased partner Mendell Stromm that gives him access to a safe house in Paris, Europe. Inside, Norman finds an Artificial Intelligence housing the brainwaves of himself and his son Harry Osborn as well as two cloning pods. Mary Jane, a prisoner of Kindred, is shocked when the demonic character reveals he's not exactly who he claims to be, and is seemingly revealed to be Gwen Stacy. Peter, in the meantime, is transported to Paris, where he is confronted by another version of Kindred...but how can they be in two places at once?, the answer is of course there are two Kindreds at play, and neither are truly Harry Osborn or Gwen Stacy, but Gabriel and Sarah Stacy, Gwen and Norman's children from the notorious saga 'Sins Past'. Using a formula bequeathed to them by The Chameleon, they have taken on the guises of Harry and Gwen to manipulate and torture Peter and MJ. It seems the revelation may very well be that Gabe and Sarah were never truly Gwen's children either, and were created at the safe house in Europe by the Harry Osborn A.I Unit, with Mary Jane's memories of Gwen and Norman's affair the result of mental manipulation by Mysterio in the guise of MJ's therapist