The third Robin, Tim Drake and Conner Kent have always had a close friendship. Throughout their time on Young Justice and the Teen Titans, they were best friends, supporting each other under the weight of the respective legacies both young men had taken up. So when Conner was killed by Superboy-Prime it was a traumatic event for poor Tim. The young Robin spent the next year traveling the world, ostensibly trying to recover in the aftermath of the loss, but deep down he could not let go of his belief that he could save his friend.
Thus Tim attempted multiple times to bring Conner back into the world despite the ethical implications of his methods.
The first major attempt was one Tim undertook on his own during the events of Teen Titans #34-37 by Geoff Johns, Tony S.Daniels, Kevin Conrad, Richard Horie, Tonya Horie, and Travis Lanham. Having acquired the DNA of both Superman and Lex Luthor, Tim began replicating the cloning process that birthed Conner. But even with his genius intellect and near-limitless resources, Tim was unable to find the correct method to bond Kryptonian and Human DNA. That didn't deter him at first, but after ninety-nine failed attempts and having no clue what he was doing wrong, Tim's rage and frustration finally boiled over as he destroyed the secret lab he had built beneath Teen Titans Tower.
Eventually, his fit of rage attracted the attention of Wonder Girl, who tried to comfort Tim in his moment of despair. Tim agonized over the loss, vocalizing how much he missed Conner. He admitted that deep down he knew that even if he succeeded in cloning his friend, that it wouldn't really bring him back. Whatever he made would only bear a physical resemblance to Conner, but would not have his experience, memories, or personality. But by that point, Tim had lost so much that he would settle for "close enough" instead of having his true friend back.
Tim eventually gave up on his plan to clone Conner. Aside from the technical difficulties roadblocking him, he realized that ultimately "close enough" was not enough for him. Tim resigned himself to try and accept Conner's death. A decision that held until Ra's al Ghul offered him the power to defeat death. The temptation was too great for Tim and he actually ended up fighting Nightwing in the chamber of a Lazarus pit over whether they had the right to defy the natural order of things when so many villains flagrantly did it so frequently. But Tim wouldn't be Batman's partner if he couldn't make the right choice, even if it was the hardest thing he's ever had to do.
Tim well and truly put aside his attempts to revive Conner. The time had come for him to accept that there were some things he could not change. And more importantly, there were some things he had no right to change. But fortunately for Tim, his deepest wish would be fulfilled and Conner was revived. Conner was placed in a Kryptonian healing chrysalis that brought him back to the world of the living. Tim was reunited with his best friend and a small part of his broken world began to mend itself as the two caught up.