Pipo is a lesser Ender, or a foreshadowing of Ender, or the Ender you get before you really get going. Like Ender, he's wise and knowing; like Ender he has an instant fatherly connection with Novinha; like Ender, he's got a way with words. When Dona Crista says, "Such a poet…Do the piggies understand that we've sent our very best as our ambassador?" (1.86), she could just as easily be referring to Ender as to Pipo.
When Pipo gets killed by the piggies, he's not reborn as a tree, but as Ender. His death calls Ender forth (Novinha asks Ender to speak Pipo's death), and Ender takes the position in the narrative that Pipo held—the wise man who acts as ambassador to the piggies and to Novinha both.
The link between Pipo and Ender casts some light on Novinha as well. Pipo is a surrogate father for Novinha. When Novinha marries Ender, she is in some sense also marrying Pipo, which means that Ender is in the roll of father as well as husband. This passage then seems to have a double meaning:
[Ender would] show [his own neglectful parents], three thousand years later, how a father should behave. Bishop Peregrino married them in his chambers. (18.229)
Straightforwardly, Ender is going to behave like a good father to Novinha's children. But, with Pipo in mind, it seems like he's also going to be a good father to Novinha herself, who remains the wounded child she starts out the book as. Novinha's story could perhaps be seen as an effort to find a Pipo/father whom she can marry. She starts with Libo—Pipo's son—but she ends with Ender—Pipo's double.