ALSO POSSIBLE: A young Japanese playitcher called Rouki Sasaki has appeared for the Dodgers like a savior. ALSO POSSIBLE: A young Japanese pitcher called Rouki Sasaki has appeared for the Dodgers like a savior.

His fastball exceeds 160 kilometers per hour, butalthough he has given off a rather fragile impression in the past. His fastball exceeds 160 kilometers per hour, although he has given off a rather fragile impression in the past.

“… in the past” would give your your second clause a a frame (of time). Additionally, “although” would convey your attempt to contrast the two ideas more clearly.

ALSO POSSIBLE: His fastball exceeds 160 kilometers per hour, but he has given off adespite the fact he seems rather fragile impression. ALSO POSSIBLE: His fastball exceeds 160 kilometers per hour, despite the fact he seems rather fragile.

Because of that, I thought his first year in the majors would end without much success, —but suddenly he is shining as a relief pitcher. Because of that, I thought his first year in the majors would end without much success—but suddenly he is shining as a relief pitcher.

“Because of that, …” follows up with your previous sentence; I believe it should improve the flow of your piece. I also added an em dash before the “… but suddenly he is…” part to connect it to the rest of the sentence, as it is a “summarizing clause”