(sorry for the non direct reply, but I haven't seen anyone post it so I wanna talk about it..!)
yes. it's revealed in the first chapter of the Kabuto novel. here's a summary:
tldr: Worm meteorite crashed into Shibuya on 19th October 1999, which was Tendou's 14th birthday.
age 3: Kusakabe Souji saw his parents get killed by Natives and see them tranform into his parents' appreance, so he ran away. he was taken in by his grandmother and changed his name to Tendou Souji. Wasn't able to speak for a while due to shock from seeing his parents killed in front of him but was starting to understand what had happened.
age 7: seething for 4 years straight so he started training to take revenge and kill the Natives that killed his parents. Thinking about his unborn sister made him seethe even more. His vowed to avenge his parents and Hiyori.
age 14: on his birthday, he was walking around Shibuya (before the Shibuya incident) because it was where his parents worked so he had a habit of walking around there on his birthday. He saw the Natives who mimicked his parents and he started following them, thinking of how to efficiently kill them both with his homemade knife. Even though he had resolved himself to killing them, he hands were sweating and trembling.
Eventually he followed them to a train station and decided it'd be best to push them onto the train tracks, and if one of them were to escape he'd just kill it with his knife. As he was preparing to do so, he saw a little girl coming up to the Natives and them calling her "Hiyori".
Tendou did not know that when the Native mimicked his pregnant mother, the fetus inside would also get mimicked. He was stunned for a bit and didn't get to push them onto the tracks and was interrupted by a DOOOONNNNN! (which is the Worm meteorite that crashed into Shibuya) blackout, train station collapsed, dust everywhere, etc
Tendou woke up after a while and found that the Native that mimicked his father was still alive so he got out his knife was ready to kill him. then cue Hiyori under rubble "Don't kill!" scene, and Tendou gets handed the rider belt by his Native father and was told to save Hiyori.
So of course he did, because she was his only sister.
"ore ga soba ni iru" and all that, Tendou put on the belt which gave him even more strength and he was able to pull Hiyori out of the rubble and basically swore that if the world turned against Hiyori, he'd turn against the world just to protect her.
following the timeline, this was technically the first grandmother quote we get to see: 「おばあちゃんが言っていた。たとえ世界を敵に回しても守るべきものがある」
("Grandmother said, even if the world turns against me, there's something I must protect.")
^first and last pages of the 1st chapter
the zukan does talk about this event, but does not specify how it was also Tendou's birthday
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