MISSION MODEFor the first collectable, "treasure chests", you just have to locate all the ones that are available in your current map. Be aware that, often, not all of them are acquirable in the very first time you play across a particular mission, and you may be required to go back to previous ones in order to grab skipped (or previously) unattainable treasure chests. Generally, you can't access these chests because they are too far away, or too high, for the skills of your current character.
The second collectable, "items"... these are usually unlocked by completing the mission, or some additional goals (represented by a divided bar) it may have. Defeating enemies also makes them drop items, and those are shown (along with the ones acquired from treasure chests) in the right side of the post-mission screen. As for the left side of that same screen, it contains items awarded as part of missions.
The third collectable, "badges", consist of up to three special badges that some levels contain, and which unlock different things. The first one unlocks that mission for the normal trial mode, the second awards you that mission for the SP trial mode, and the third one fully unlocks that level for the multiplayer "Mission Mode". That's the information I include to the right of the mission number, ranging from "---" (a mission which contains no badges at all) up to "OOO" (mission which has three badges), together with the likes of "-O-" (mission which only has a SP trial badge).
Saix is one of the default playable characters in the Mission Mode section of Kingdom Hearts: 358/2 Days. On opening Mission Mode, you are given a character select screen that shows you the stat balance for each character. Below is the following information on Saix's basic stats.
Max HP: 7
Crit Bonus: 10
On beginning Mission Mode, you are prompted to load from a save slot. All levels, panels, and unlockables opened up in Story Mode will carry over to Mission Mode. You can, however, choose Guest Play instead, which is a good option if you have no save data or are at the very beginning of the game in Story Mode. You cannot save while in Guest Play, and you are limited to the original 13 members of the Organization as playable characters (still a wide array of choices, of course). You start off with 5000 Munny and 300 Heart Points, which you can use at the Moogle Shop to buy or synthesize panels. You're also given a selection of panels to equip in one of 25 open slots. Your panel inventory includes items, equipment, magic, abilities, and gear, but no rings or synthesis items.