Minor spoilers ahead for characters and plot. Also, it's been a while since I've read volumes 1-3, so most of this will be based off of volumes 4-5. This series, in a sentence, is a feels-good isekai power fantasy-and if that's up your alley, then I would definitely recommend this series. Characters: Characters are more or less pretty generic. The main character has little personality outside of being a normal Japanese isekai protag, and lacking a lot of common sense because of his "perfectly normal" training arc. The white mage is similarly fairly generic, her quirkiest quirk being that she's a heavyweight (drinker). I know thatI keep saying generic this and generic that, but I honestly don't know how else to describe them. Maybe one dimensional is better?
Regardless, this doesn't make their interactions any less fun to read. In a era of manga/anime/novels with so many big brain characters, each playing 5d chess with each other in every interaction, reading about these straightforward characters is relaxing. You have an easy time getting to know them, and quickly start to root for them in whatever chaotic situation they're in.
Of course, there are also some more complex characters (though they're still fairly simple, so don't go expecting too much). The lich that the protag first meets is always a great addition to any scene, and the innocent eldritch abomination adds another flavor of chaos.
Overall, characters by themselves are fairly flat, but when they interact, each person adds a layer of fun to the conversation that feels cozy, enjoyable, and substantial.

Plot:
The overarching plot is essentially just the protag trying not to die to the garbage god that summoned him into the isekai world. The god keeps throwing demon king tier enemies at him, but to no avail (because he's op). Things keep escalating until the group that are basically the keepers of the world start sending out members to hunt for him.
The plot is nothing exceptional, but it's definitely not bad either. Of course, battles are always pretty easy, since the protag is op. But, there are still fun little twists that spice things up, predictable they may be. Each volume also introduces new areas and, more importantly, new characters, which is what I personally look forward to most.
If you're looking for complicated, multi-layered plotlines, then this probably isn't for you.

Art:
This part is subjective, so I won't hold back in saying that I love the art. The colored pages and cover art are always gorgeous, and the character designs are appealing. The black and white pages are a bit lacking compared to the colored ones, mainly due to fewer details, textures, and backgrounds, but are still nice to look at. They're also well interspaced-sometimes there are too few drawings in light novels, and it's unsatisfying when you can't see a really cool or cute scene in picture form, but I haven't felt that way while reading this series.

Overall:
Disciple of the Lich isn't for everyone-heck, I'd argue that most people wouldn't like it. The combination of op protag, lack of tension, and simple plot just doesn't do it for lots of people. However, if you do like these things, or you just need a break from other high action high stakes series, and you just want to read silly turn-off-you-brain fun and to root for your favorite characters, then this series is a great place to get that. Happy reading~

Rating (enjoyment): 9/10