after reading all that manga I don’t understand what’s happening.... (specially about Sumi and renko, like... if renko’s dead, sumi's dead too? And if that was a dream, did Sumi died in the dream? I really don’t understand that sad paradox.... and trust me, i DID read the entire thing...
i guess it's based on the theory that Sumireko was Renko in highschool and stuff. In this story she's dying and has this random dream over and over again (yeah i call it random so i won't have to think much of what happens during it)
I’m not an native English speaker, but here’s my explanation of "Good-bye Utopia".
Renko had been in coma since the beginning of the story. Of course, she didn’t know what exactly is happening, but down in her subconsciousness, she knew that Maribel's about to be separated from her, which was the only thing she knew. Deep in Renko's mind, her determination not to leave Maribel led her dreaming about herself being a powerful psychic, Sumireko, and try to save Maribel every time in the looping nightmare.
But the nightmare became so ridiculous, because Renko subconsciously knew that it's impossible for her to keep being together with Maribel. Until the real Maribel interrupted her looping nightmare, Renko finally “admitted” what's going on with her---- The whole Sumireko's looping nightmare stuff was Renko herself's nightmare all along, it was her mind’s desperate attempts to "save" Maribel.
After hanging out with Maribel inside her dream, Renko finally felt comforted, and finally facing the truth of her time spent with Maribel had to come to an end. With a bang, she ended her dream, her nightmare. In the reality, she ended her desperate attempt to stay alive, marking the end of her suffering, and the start of her eternal peace.