In Attack on Titan's recent manga chapters, Zeke tells Eren that his father had lied to him and that he was unaware of the truth. Why is Zeke interested in Eren even when he can control titans and transform into one?
Update: This was further backed up by the events of chapter 87
What I think is that, according to latest manga chapter (86)
Zeke and the other warriors are specifically trying to retake the coordinate from Grisha (and now Eren). He is also Eren's half brother (not step-brother) but it would appear that family is not an important bond to Zeke, as he betrayed both of his parents.
According to latest manga chapter, 86,
Zeke is older step-brother to Eren. He was child of Grisha from previous wife, in "outer world". Full motives of Zeke are still unknown, but, most probably, he fills empathy to Eren, thinking that his brother was also used as tool by Grisha in his quest for revenge.
Spoiler alert. In the story, it is said that Zeke ...
... was brainwashed by Marleyan government when his parents, Grisha dan Dina Yeager, sent him to be a spy by joining the training to be a warrior. Zeke ended up believing what their father told him was untrue and ended up betraying them.
So I think he told something like that to Eren because ...
... he believed what the Marleyan told him was true and he wanted to tell Eren that everything he knew from their father was wrong. Perhaps he wanted Eren, the current holder of The Founding Titan and The Attack Titan to be Marleyan allies. That's their goal that we know so far.
Since the story hasn't ended yet, another possibilities might happen later. Zeke is surely full of secrets.
Zeke still wanted Eren even he can control and transform titan perhaps because ...
... the power of the founding titan or what they called as the coordinate is considered thousand times more powerful when it comes to control people.
Remember back then when Zeke ...
... confronted Mike in Wall Rose? The titan still crushed Mike even Zeke told it not to. His titan's controlling power was so much weaker compared to the founding titan when the first king managed to command millions of colossal titan to make the wall and modified the memories of people those living inside it.
And I guess Zeke's power of controlling titans ...
... has something to do with the royal blood that flowing inside him, since it's said that the previous holder of beast titan didn't have that kind of ability.
Zeke Yeager is Erens half brother. His parents are Grisha Yeager and one of the kings royal blood predecessors(who later becomes the smiley titan that kills Erens mother and Hannes). He betrayed his parents and got them caught.
To understand why:
Basically, there's an island where the humans are thriving outside the three walls. these are the original people. the people protected by the three walls (Eldians) are meant to have betrayed humanity, so they were cast away. The ones that remained on the main island were treated like they were dogs. After the military in the main island killed Grishas sister by feeding her to the dogs, Grisha swore to kill all the Marley (people on the main island that discriminate the Eldians). They finally realised that they need the coordinates from the king (Whos behind the wall). The Marley want the coordinates too, so they assemble Eldian soldiers to go and get the coordinates. The soldiers will be given the power of the eight titans the Marley government had. Grisha planned to use his son Zeke to get into the army, become a titan, and then steal the power of the king for the Eldians, but he rats them out.
Zeke Yeager, Reiner, Annie, and Bertholdt use their titan powers to try to get the coordinates from Eren. He doesn't want to eat Eren, he wants Eren to realise that the Marley government is the true Government and that both he and Zeke were used by their parents.
and that is why Grisha is obsessed with Eren
An update with the latest information from the recent chapters.
It was revealed in Chapter 114 that
Zeke's true plan was Eldia's euthanization. He believed that it was the only salvation for the Eldians. He also believed that without Eldians, there would be no more suffering in the world.
Thus, he sought Eren in order
to convince Eren to take part in his plan of euthanasia of the entire Eldian race. He also tried convincing Eren that it was the only path to 'salvation'. Marley's orders to recover the Coordinate was a perfect cover for his true intentions as none would suspect why he's interested in Eren, the current wielder of the Coordinate.
He must have
succeeded because as seen in the recent chapters, Eren is now in league with Zeke after they talked during Eren's infiltration to Marley. Personally, I still have doubts on whether this is all a ruse or something. Titan-shifters can see memories of the future and it's possible Eren might have seen or known something via these memories that made him side with Zeke.