Gemini Saga is one of the most popular Saints in the franchise, and for years he was the first DPS almost everybody built. His plan was always simple — Galaxy Explosion and nothing else — and it stopped being enough long ago: he became a PvE unit for new accounts, with no place in PvP. Destiny Reshaping changes that completely.
The new mode does not add mechanics, it rebuilds his base stats, exactly what he needed: Saga was always a unit with a small health pool and weak defenses, and both problems disappear. He now has more HP than any tank in the mode, better physical defense and extra Speed, so he finally gets to play. Since every Cosmo scales from that new base, his damage grows with it, and the Reshaped Body of Dilemma does the rest by compressing the damage of his previous explosions: the first area attack is already scary, and each following one hits harder.
That is why he is one of the best investments in the game right now: in the Chinese server he is already being banned in Platinum and Diamond, and even a casual account with average Cosmos can climb with him because he asks for very little. If you are a beginner or a mid-level player, this is almost a free carry; if your account is already full of modern DPS, he is less urgent but still strong.
His synergies are the classic ones. Aquarius Mystoria is his best partner, because he links with the ally with the highest attack (now always Saga) and covers him with a shield that blocks a fatal hit. The double Saga draft also works: Surplice Gemini Saga weakens the whole enemy team inside his dimension while the original does the damage.
His weaknesses did not disappear: he is not immune to control, and a well-timed freeze still shuts him down before he can stack. He also needs several turns to reach his ceiling, so he is better in long fights than in turn-one races. Is he worth investing in? Yes, especially if you played him years ago and miss him — this is one of the biggest upgrades of the season. Just remember that the last level of the mode asks for the Divine Ichor you only get from events, and the difference between the last two levels is small, so it is safe to wait.