There are mainly three ways to recruit characters. You can use the blue-ish computer to access the scout tree and recruit characters from there, you can use the tokens you get(the blue, red and yellow coins) and get characters from the machines that look like gumball machines that are in several different places and finally you can recruit characters from other teams you face using the green computer.
Also, you can get some characters through beating them in random battles, at least one which you have to follow through various locations (near water) and speak to him, characters passed with the Super Link system (Inazuma Eleven 2 data to 3) and password unlockable characters, which are divided into three groups.
1-The ones you can found always in a point. They ask you always the same question and you have to anwser it password-wise to be able to get them through direct battle or by another of the previous systems, although there is a group of 5? players that you must answer all of them to unlock their profile in the Binder.
2-The "mobile" password, which involves getting into a japanese website through a cell and getting random generate passes depending on your username. Using the original method is quite difficult for non japanese readers, so you just input your savegame's name in here http://oupo.github.com/tools/inazuma3-password.html (Thanks again Koji) and it generates passes for all the unlockable players (use wikipedia's entries "Hiragana" and "Katakana" for assistance or copy-paste of the letters)
3-And last, the hardest one right now. There are two secret players related to the IE movie, which can be unlocked pre-ordering a movie ticket and using the chart included to input the codes they ask you (for old gamers, the Monkey Island protection system). Cwiboon put this link http://www.thaids.com/attachments/month_1007/10072722567f7b2b1c9cbd9f5e.gif for getting the codes without buying anything, although it's a very fast gif, so it could be a bit difficult catching them.
PD: If you are curious about the password ones, in this post it's explained how to access the game's feature and also all I explained here ;) http://www.gamefaqs.com/boards/989987-inazuma-eleven-3-sekai-e-no-chousen-spark/55525789 PD2: And I nearly forgot, you have some other players unlocked from the extra matches, usually achieving an S rank in a concrete branch.
To add on, the Scout Tree (bluish machine) that NewZuperEskimo mentioned, have locked characters.
There are 3 types:
- The first type will automatically unlock you reach a specific chapter of the main story and/or fulfilling certain conditions that the lock tells you to do so.
- The second type are the locks at the perimeter/border of the scout tree. Connecting to WIFI and updating from there will unlock them. A new character will be unlocked once every Thursday, so just connect online for your bonus goodies per week.
- The third type involved that 4x2 locked rectangle. It is said that you will need to perform DS wireless download. This is still a mystery to me.