So you can customize your party by concentrating on strong combat characters, or strong spell casters, or characters that are good at helping you solve puzzles. Or, you can make a balanced party of several different types. However, each character in your party does have a specialization in one or more areas, as well as having a unique set of skills in addition to some skills that are shared. For example, while all the characters can use the Spell Cards, Selina is very good at it, while Runner is very poor at it. However, Runner has great physical strength and armor to make up for this.
Characters use an experience point and level based system to advance.
When characters go up in level, they often learn new skills. Other skills are based on the equipment that the character has. The next time she is in combat, that skill appears in her skill list. Right now the game isn't really segmented into 15 visible chapters, although this may change. The design was created this way to keep it under control.