Red-haired Adol Christin is a wanderin' man, so it's appropriate he'd find his way on mobile eventually. DotEmu just released Ys Chronicles II for iOS and Android, a direct follow-up to Ys Chronicles I -- which is also on iOS and Android.
If you put the games together, you more or less get Ys I & II Chronicles, a compilation that exists on a pile of systems. If you don't want to play mobile versions of the game, you might consider grabbing the I & II Chronicles on Steam, where it's on sale at the time of this writing.
However, I can't tell you all about the nuances between each release of Ys I & II Chronicles. To quote Ys fan DIO-sama on their review of the Steam collection: "What we have here is an enhanced port of a PSP port of an enhanced edition of a windows remake of a combined turbografx-16/pc-engine remake of a pair of games on an ancient Japanese pc."
When Ys Chronicles I and II are studied on their own merits as mobile games, however, you find a couple of very decent RPGs that are admittedly on the strange side, especially if your entry point into the expansive Ys series has been one of the newer games that allow you to swing Adol's sword under your own power.
See, neither Ys Chronicles I or II designates a button (or a portion of the screen) for hacking-and-slashing. Adol goes to war with baddies by literally running into them and delivering some rapid-fire stabbing action. It's a bit hilarious, and frankly, kind of neat.
It's also a mechanic that defined the earliest Ys games (it's called the BUMP system!), so no, fighting enemies by running headlong into them isn't new to the mobile adaptations.
That said, bumping into demons is a good way to get the job done on mobile. It's one less virtual button to worry about, and less chance that your thumb will miss its target in the heat of the moment. And while Ys Chronicles I is on the rough side, Ys Chronicles II smooths things out with clearer enemy cues. You know precisely when a monster is going to take a piece out of you, and you can dodge accordingly.
There's also an Easy Mode, which helps ease ("Ys!") you into the action.
Ys VIII is coming to PlayStation 4 and PSP this summer (in Japan), so now's as good a time as any to join its small but growing North American fanbase. Question is, should you start at the story's beginning with Ys Chronicles I or II for mobile, or jump right ahead to something more modern, like The Oath in Felghana (which is also on sale on Steam)?
As a relative Ys newcomer myself, I enjoyed the time I spent with Ys Chronicles I and II on mobile, though I found II far more polished than I thanks in part to a much improved translation. Again, the bump system works surprisingly well on mobile phones, and there's something compelling getting right up in your enemies' faces. As good, solid mobile RPG experiences, the Ys games are good investments.
If you're simply not interested in playing a mobile version of Ys, or if the bump system intimidates you, go ahead and get your feet wet with something more modern, like Oath in Felghana, The Ark of Napishtim, or Ys Origin.
Which -- guess what! -- is also on sale on Steam.