Are there any characters I can miss in this game? If so, which are they? I know about Tir, so don't worry about him.
(Insert witty signature here)
Missable Characters (Mostly for people wanting to get all 108 Stars of Destiny and the Best Ending)
Mukumuku - Mukumuku can't exactly be missed, but can be rather hard to recruit later on in the game. You can recruit this squirrel near the beginning of the game. When you first go to Kyaro Town, before heading into your house, follow the path on the right side which will lead you to a large tree. Simply 'talk' to the tree 3 times and Mukumuku will reveal himself. If you missed him here it's alright. Later on, when you are heading to Greenhill to save Lady Teressa, make sure you have a single slot open in your party, you can recruit Mukumuku by walking between Greenhill and the Muse/Greenhill border. You don't have to recruit him at that point, but it is said that that is the easiest time to recruit him.
Clive - When you enter Muse for the second time (After the Sindar Ruins scene at White Deer Inn), head to the very right of the city and then walk up, going through the ally. When you almost reach the top, a lady named Elza should stop you. Accept the package and keep going. Then Clive will stop you and ask if you have seen a lady named Elza. It doesn't matter which one you answer. Later on, try to leave Muse and Elza will stop you and take the packages from you. There will be a scene with Elza and Clive. Then later on in the game, when you reach South Window, you will see Clive in the shadows facing towards the exit. But before you actually recruit him, you must have already visited Lakewest. If you missed activating the scene in Muse with Elza and Clive, you won't be able to recruit Clive.
Gilbert - During the map battle near muse, where you have to stall Highland for a few days, you will see Gilbert on the battlefield fighting for Highland. After Highland has moved, Viktor and Flik will mention about Gilbert. Then Viktor will use his Fire Spears on him. It should damage him (He will have one sword stabbed into his unit). If he does, there will be a conversation between them and Gilbert will soon switch sides. It won't say "Gilbert joined your group", but you will have Gilbert recruited. After this, make sure he doesn't die (If he does, make sure it only says that he is injured and not died in the battlefield).
Futch and Humphrey - When you visit Rockaxe to create an alliance, go to Highway Village and activate their scene. After the event about Kent going missing, agree to help and then go and complete their quest. After beating the Harpy and saving Kent, Humphrey and Futch will be recruited. You can do this also in the beginning of the game using the Matilda Glitch (See next section). It is best to do this now since afterwards, you won't be able to go back to Highway Village until after the point where you need to have 108 Stars of Destiny, which is near the end of the game.
Jess - During the Tinto event regarding Neclord, you can actually miss recruiting Jess if you didn't accept his apology.
Taken from JHawk's sticky topic. There's also Ayda; when you get the listening crystals, use one of them on the gryphon and don't use them on more than one octopus(?).
"Je pense, donc je suis"