She Eternal, He Immortal

Hancock/oh my goddess type immortality (paired immortals, can only kill each other and they die with them)

Sailor Pluto paired with Ranma Saotome

Pluto chosen as guardian due to her immortality

After the Sliver Millennium's fall, Sailor Pluto decides to kill her other half and thus die but has to wait for him to be born.

Queen Serenity appears before Pluto as she is about to kill Ranma and says that Ranma must command her to kill him with the knowage that her will die too

Pluto can only see Ranma's present (due to his chaotic nature/aura)

Pluto is horrified by Ranma life

By the time Pluto confronts Ranma she has found happiness with the reincarnated senshi but still feels that Ranma should choose

Ranma refuses as she is happy

Future Ranma watches crystal Tokyo from a distance. As a being of chaos he cannot enter but guards it secretly. (though he hate Neo-Serenity naivety on good and evil)

Ending with Pluto watching Ranma from the time gates and sheds a tear of regrets before she is called away by her family.