Kazuma Asuka went disappeared in a Zero Drive navigation test into a beam of light. He was mentioned the first time by Masami Mishima, who was a close friend of Kazuma in around the same time and they were flying together until this accident happened (Kazuma's disappearance). He only appeared in flashbacks and once in Asuka's illusions when Dyna was frozen by Reicubas. He was presumed dead but in the final episode, he was revealed to be alive, joining his son when the latter also get sucked into a wormhole and travel towards the space-time continuum.
Ultraman Dyna Novel: Zero Drive to the Future
In the Ultraman Dyna Novel, a novelized retelling of several key events in the series by Keichi Hasegawa, the main writer for the Dyna series, Ultraman Dyna is revealed to be the transformed state of Kazuma Asuka. The explanation given is that Kazuma's presence into a 'dimension of light' and the drive system he was testing, somehow transformed him into a giant of light. He was stated to have then spent several years between the end of Tiga and the start of Dyna, fighting evil across space. However, upon sensing his son's crises, he flew across space and merged with Asuka to save his life.