Bemstar is a recurring kaiju in the Ultraman series, making his debut in episode 18 of Return of Ultraman.
Return of Ultraman
Bemstar came to Earth looking for cosmic energy, devouring a space station upon his arrival. Ultraman Jack soon came in to battle Bemstar, but his attacks did no damage to the monster. Weakened, Jack went to gather more energy from the sun, but wound up being pulled into it. He was saved by Ultraseven, who gave him the Ultra Bracelet. Now fully powered up, Jack went back to Earth and fought Bemstar, cutting off his arms and decapitating him with the power of the Ultra Bracelet.
Bemstar appears in the flashback in episode 25 as one of the members of Alien Empera's Empera Army. Bemstar is revived by Yapool as Reconstructed Bemstar and sent to attack Earth. Ultraman Taro and ZAP both fought against the monster, but ZAP's weapons were unable to do anything while Taro was beaten down by the revived kaiju.
ZAT and Taro later fought Reconstructed Bemstar again, faring much better than they had before. Yapool sent down Verokron II and Sabotender to assist Bemstar, but they were destroyed by ZAT and Taro respectively. Bemstar was then destroyed with two missiles to its Gorge.
Ultraman Mebius Gaiden: Hikari
A Bemstar attacked Ultraman Hikari as soon as he left Earth. Hikari fought back against Bemstar, but the kaiju easily overpowered him. However, Zoffy showed up to assist Hikari and together the two were able to defeat Bemstar, with Hikari using a Hikari Light Stream to destroy him. Hikari later noticed another Bemstar flying to Earth and attempted to stop it, but Zoffy convinced him not to, telling him that Mebius was on Earth and that he needed to have faith in his comrades.
Bemstar consumed a space station upon his arrival, just like he had in Return of Ultraman. Though fearful due to stories he'd heard about it, Mebius went to fight the kaiju anyway. Bemstar attempted to defeat Mebius by sucking the Ultra into his Gorge, but Mebius managed to free himself and later destroyed Bemstar with the variable size blade.
Ultra Galaxy Mega Monster Battle
Bemstar attacked ZAP SPACY upon their crash landing. Rei summoned Gomora to fight Bemstar, and later summoned Litra to assist him. Litra transformed into her phoenix mode and fired a Fire Strike at Bemstar. Bemstar absorbed the attack, but this left him wide open to Gomora, who destroyed him with a Super Oscillatory Wave.
Ultra Galaxy Mega Monster Battle: Never Ending Odyssey
A Bemstar attacked the Pendragon as it was flying through space but was scared off by the Pendragon's arsenal and killed when Dail used the Pendragon's Anti-Asteroid Cannon to obliterate it.