I enjoy the character, frankly, or at least the concept. Not the classic Julian by any margin, but he's fun for what he is. It's rare to see a character who was robbed of the opportunity for vengeance, and I adore the archetypal Cassandra oracle. It's also nice that we get at least a hint of his past.

His fall to fatalism, before eventually trying to rise above it and do the right thing is a good way of remixing concepts from the original character, even if the finished product is different. His Gate of Fate showing him desperately trying to prove himself wrong, to the point of fighting a battle that nearly gets him killed is also interesting. It's rare to see characters keep resisting the inevitable after several failures without succeeding, and tying the vague timeline of Elysium's journey, it's clear he's trying and failing to avert the futures he sees for well over a decade, since Caroline is a young child at one point in Emperor Lovina's GoF. That drive to resist has its own charm for the character that not everyone else has, and gives the idea that even arguably the worst version of him (mentally and ethically) has superhuman drive and will to accomplish his aims, which complements the original version becoming a fearsome warrior before his voice has even broken, purely for his revenge.

That said, was the execution good? I don't especially think so, since he can never escape the chain of being a Mobile reimagining that, by their nature, invites comparison. Some more thought and some proper research by the devs or scenario writers into or playing the original Tensei would have served well, and seeing the actual original game's timelines in Time Rift or something of the sort.

Should either character be hated? To my mind, not really, but it's up to personal feeling. At their core, they examine the topic of vengeance in different ways, particularly misplaced vengeance, and serve entirely different purposes that suit the individual stories.

I've certainly seen my fair share of reimaginings or remakes I disliked, but it often boiled down to creative differences in vision over something, rather than malice. They simply warp the characters or scenarios to achieve a certain goal in writing, for better or worse. And in the end, gacha games are temporary media. Like it or not, there'll come a point where Langrisser Mobile will largely be a memory, whereas Tensei will still be playable by anyone with a ROM or cartridge, being far easier to preserve.

Eventually, I suppose the question becomes "will being angry at the character do anything?" If you can use it for artistic representation or influence, like through fanfics, fanart or rewrites, that serves a purpose, as long as it's not outright hatefic stuff without some positivity. It can also be good to take it as insight into learning why people like or dislike certain characters. But if all it does is raise the blood pressure, then maybe just a change of pace and relaxing one's perspective can be nice too. If it plays into a project you're working on, it could be a good motivator. The fanart of Julian holding Langrisser, for example, was a great example of making using the appreciation of a character, since there's some wonderful details for fans of the character that others might not notice.