"Life at best is bittersweet" is a phrase which was said by Darkseid in Miracle Man #18, written and illustrated by Jack Kirby. It is often used as a quote to illustrate Kirby's outlook on life. In modern references, it's more associated with Kirby King's letter to Donatello.
The phrase implies that life has its joys, but also many sorrows, and that even the best-lived overall life experience includes both good and bad memories, averaging out as a bittersweet experience. True to the phrase's assessment, the lives of most core characters in the TMNT franchise tend to be very complicated and laden with personal baggage, as they struggle either with living as mutants on the fringes of society, or as humans whose lives are deeply embedded in the secret world of mutant friends they can't talk about, or as people in general facing lives of frequent hardship and violent conflict.
The phrase first makes its appearance in conjunction with TMNT at the end of Kirby and the Warp Crystal.
The phrase first appears in Kirby and the Warp Crystal.
It is the last message Donatello ever sees from his new friend Kirby, who had just become permanently stranded in another world when a portal back to Earth separated them. Don read the message with the knowledge they would never see each other again, and he might never know Kirby's ultimate fate.
Leo, 15: So... Will our lives remain as good as they are now?
At the time of this story, the 15-year-old Leo had been trained in ninjutsu, but had not yet faced the Foot Clan for the first time, and was still relatively innocent and inexperienced. By the time he would be 45, he had already known extreme suffering, loss, grief and regret from a life filled with violence.