Other Worlds: the early Isekai of Aura Batler Dunbine

Aura Battler Dunbine is an early 1980s Anime Isekai in which a Motorcyclist from Japan is harassed by idiots in cars on a highway ccausing him to jump his motorcycle over the car ahead of him only to intersect a dimensional doorway in the air. He awakens in the lower world which is populated by medieval european humans, monsters, and various fairy folk, along with ufo and mecha technology thanks to other recent humans from the upper world. The local lord (an American from California) has been enslaving a selkie (water-woman) to use her power to open the gate and snatch folk and technology from the upper world. A number of upper worlders are pulled through and the hero and companions- including an American who refers to the hero as 'Jap'.
A 'rebel' faction fairy incites the hero to stop enslaving the selkie to no avail at the gate site refered to as Byston Well and the upper worlders are taken to the lord's castle. On the way there they see a UFO which is a product of the local lord's greatness. Axxnd on arrival are treated to a gladiator event in which a hero uses a powered armour/mecha to butcher a large creature for the crowd's entertainment. That night a tiny rebel winged fairy (refered to as ferario) again attempts to make contact with the hero inciting him to help.

In further episodes there is revealed to be conflict with rival nobles who have slowly acquired technology including powered armour/mecha.
Next day during mecha flight training the hero fights a red haired rebel faction mecha knight who gets away. He recognizes her later at a meeting between a rival noble and his current 'lord'.
A noble woman travelling in a coach drawn by horses is killed when her coach is attacked by mecha having been mistaken for the rebel faction lord.

Its starting out interesting, and the red haired lady mecha-knight (an upper worlder named Marvel Frozen) reminds me of a similar anime character in the recent RE: Creators anime. She seems to have become a common trope heroine making her way through many such anime.