Completely fill the character notebook.
Talk to an NPC to add them to your character notebook. It's easy to find everyone in the game to do this with as long as you check the towns thoroughly, except for one person: Doctor Klaus, number 43 in the notebook. When you first enter Minea, he'll come rushing out of the clinic, heading for the exit of the city - make sure you catch him, because this is your only chance to talk to him in the game!

Unlock all details for all non-boss bestiary entries.
This isn't as easy as filling out the character notebook, because killing each monster once isn't enough - you need to kill lots of them in order to fill out the details for their HP, STR, DEF, EXP and GOLD. It can be a bit tedious and grindy to do this since you'll often need to keep running around the same area, trying to get the monsters you're cataloguing to respawn so you can kill them again, but thankfully it doesn't take too long. If you set Monster Stats to Display in the options and make sure you have the information display on in-game (tab on a keyboard, LB on a 360 controller) then you'll get info on the last monster you killed at the bottom right, and as their different stats appear that means they're appearing in the bestiary - but it doesn't show their EXP and GOLD, so make sure you keep checking the bestiary when you're trying to get those. Oh, and be sure that you've drunk the Bestiary Potion! You can get it from a treasure chest that isn't locked early in the game, between Minea and Zepik Village.

Complete Ys I on any difficulty.
This one's self-explanatory. If you beat the game on Nightmare difficulty, you'll get both this and the Into the Sky achievement.

Complete Ys I on Nightmare difficulty.
This one's self-explanatory too, but it's no easy feat! I'll admit that doing this had me throwing the controller to the floor a few times, especially during the bosses! I recommend saving often and healing whenever you can (the Healing Ring is very useful for this when you're in dungeons, just make sure you stay alert in case any monsters come after you whilst you're standing still to heal), because death can come extremely quickly on this difficulty and you don't want to lose too much progress when that happens. As for the boss battles, well, you can't heal or save during them, so make sure you're prepared, and some of them may take you quite a few tries before you learn the best way to take them down. If it gets to be too much, I found that looking up a video of the boss being beaten on Nightmare difficulty was helpful. Best of luck! You can do it, it just requires a fair bit of patience and perseverance!

Complete Ys I Time Attack.
Time Attack is unlocked when you beat the game on any difficulty, and it's a boss rush where you must defeat every boss in the game without dying once (it doesn't involve playing as a cute brown critter in an Adol bumper car, unfortunately). Although you're being timed, you don't need a good time to get this achievement, you just need to complete the mode. To be honest I found it to be easy after beating the game on Nightmare, since thankfully the bosses in this mode are set to an easier difficulty than that - if they weren't, that really would be a nightmare!

Receive a free Short Sword.
When you begin the game, talk to every NPC in Barbado Port (watch out, some of them are easy to miss, like the guy on the roof of the Militia Headquarters), then talk to Slaff (he should be inside the aforementioned headquarters) and he'll give you a free Short Sword.

Receive a free Talwar prior to entering the tower.
After returning the Silver Bell to Mayor Robels, go back to Barbado Port and revisit Dr Bludo's clinic to see Luta Gemma (so you can get a later achievement), then go and see Slaff in the Militia Headquarters again and he'll give you a free Talwar.

Drive a hard bargain with Pim.
At Pim's pawn shop in Minea, select an item to purchase and when asked if you want to buy it, choose "No". Do this nine times and Pim will discount his entire inventory by 10% for the remainder of the game, earning you this achievement. This also works for selling him things, it'll make him pay you 10% more for them.

Confuse and astound the townsfolk.
Simply use a Mirror anywhere to get this - it doesn't actually have to be somewhere where townsfolk can see. I actually recommend using one for the next achievement, so that's convenient! You can buy one from Pim's pawn shop.

Level-up Feena.
Yes, when you're escorting Feena out of the shrine, she can actually level up by killing a monster! But how can you get her to do that? Well, save before you do this, because it might require a few tries. You need to push her into a monster so that she can attack it without it hurting her - the same rules of bump combat that apply to Adol apply to her. I would recommend using a Mirror to freeze the enemy in place first.

"Measure" Feena.
When Feena is following you inside the shrine, take her to the short dead end corridor on the far left, near the entrance, and push her up against the wall at the end of it. You did it right if she says something about it being cramped. You pervert! I'm calling the police!

"Measure" Feena in the creepiest way possible.
Do the same as you did for the last achievement, but this time with the Mask of Eyes equipped. Weirdo!

Rescue Luta Gemma from monsters in the field.
If you went back to Dr Bludo's clinic after giving the Silver Bell to Mayor Robels so that you could see Luta Gemma leaving, he'll now be on the western part of the field, south of the Roda Tree, under attack by monsters! You'll know you're in the right area if you can hear combat noises. Defeat the enemies that are attacking him to save him and get this achievement, along with 500 EXP and a free ride back to Zepik Village. If you talk to his wife after doing that, she'll say something interesting...

Listen to Reah recite her poem.
There's a locked chest on the field. After you get the Treasure Box Key from the shrine, you can open it and find a Piece of Paper inside. After you give Reah her Silver Harmonica back and she plays it, equip the paper, speak with Reah again and then use it before walking away. If you did it right she'll say it's a poem she wrote and thank you for giving it to her, then she'll recite it when you leave the area - it seems Adol doesn't have to listen to it like the achievement picture would suggest!

Make Dogi break through three different walls.
When you get to the sixth floor of the Tower of Darm, there'll be a trap that will get you thrown in a dungeon cell, but Dogi will break through the wall to rescue you! Then he'll give you the Idol which you need to give to Raba on the third floor (use the Mask of Eyes to find his room) in exchange for the Blue Necklace, which will allow you to avoid the trap if you equip it. But if you don't equip it, you'll get thrown in a cell again, and Dogi will have to rescue you again - do that twice more to net this achievement.

Max out your money.
Getting 99,999 gold might sound daunting, but if you're going for the Monster Masher achievement, you'll actually get it pretty quickly in the Tower of Darm. Don't worry about saving your money earlier on in the game, because you'll be getting plenty that you can't spend when you're in the tower! If you're not going for Monster Masher, just kill lots of enemies there, it shouldn't take too long.

Master every achievement in Ys I.
Self-explanatory! Congratulations once you get this! And thank you very much for reading my guide, I hope it was helpful! If you have any feedback or questions about it, please feel free to leave a comment.