Spoiler

Not sure if someone mentioned it already but here it goes. Scroll down

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I found this on youtube.

Remember when Ariadoa the Sealing Goddess said that there was a world which she was unable to save? Apparently the hero she summoned was Seiya (yep that Seiya). Turns out he wasn't that overly cautious from the start but is actually a bit reckless. Still The B-difficulty world's demon king was defeated thanks to the hero and his party but they did not expect that there was a hidden stage 2 after it. All the party members were already exhausted due to the previous fight as well as the hero. Cause of that they all perish (including Seiya's unborn child in his wife Princess Tiana) and Aria was unable to complete the task of saving that world. Seiya returned to his previous world with no memory of what happened in the B-rank world but before that he swore to himself that if ever he was summoned again that he need to be 'cautious' hence the Seiya we know (It was due to his previous summoning that his stats were high at lvl1 when Rista summoned him). Aria was overcome with guilt that she decided to reincarnate Seiya's wife Tiana into a goddess, Ristarte. As mentioned in the comment below Ristarte = Restart