Resurrection in D&D generally has a few limits. First, is in terms of how many years a person has been dead. A caster is limited in resurrecting any dead person by 10 years per level. (A level 7 cleric can resurrect someone who died 70 years ago, but not 80 years.)
Second, you also need their body intact.
Using Wish, you can circumvent the second limitation. One Wish to recreate the body, and then a second Wish to resurrect it. I'm not sure if Wish can directly circumvent the first limitation. It definitely can't in 5e during my last campaign. We had to use time travel then use wish to bring back someone dead from thousands of years ago, while also replacing their corpse with an identical one to preserve the timeline.
But anyways, I'm not sure what caster level Areru is considered to be. There's a thing called Epic Levels, which means there's a mechanical way for a D&D character in certain editions to essentially level endlessly, and he also is the reincarnated form of someone who had a Gamer power.
Given the setup implying he'll try to resurrect Ryoko, and she's been "dead" (but not really) for 700 years, it would imply, by D&D leveling alone, Areru is at least level 70. Assuming that first limit with Resurrection applies here.
Queen Serenity died ten thousand years ago. Areru would have to be level 1000 in D&D terms for him to not be limited there.
There's also, of course, the matter of if the soul actually wants to come back to life. If it's properly moved on and settled in the afterlife, or worse, reincarnated already multiple times, then, tough luck.
It's hard to say if the image of Queen Serenity we see in the anime on the Moon is her dead spirit or just a hologram. The manga explicitly mentions her essence being in the Moon Palace's computer databanks, so it could just be a recording of her rather than her actual soul.
There's also the matter of how she died. She gave up her life using the Silver Crystal to ensure everyone who died in the Moon Kingdom reincarnated on Earth. That amount of magical mojo that took her life as the cost might prevent a resurrection spell from working.
