Secondary Skills

Thievery:
Lets you pick locks on chests and doors. This is the only skill which has a rating.
Robbers and Ninjas start with it, and no other characters can learn this skill.

Swimmer:
You can swim in shallow water (light blue). All characters must have it.
Humans start with it.
Rivercity (19/23): 100 GP
Castleview (29/17): 250 GP

Pathfinder:
You can walk through a dense forest. At least 2 characters must have this skill.
Rangers start with it.
Vertigo (25/26): 2.500 GP
Castleview (29/27): 1.500 GP

Mountaineer:
You can climb mountains. 2 characters must have this skill.
Rivercity (30/30): 5.000 GP
Darkside (B2 13/14): 5.000 GP

Cartographer:
Enables the automapping feature. 1 character must have it.
Sorcerers start with it.
Vertigo (8/16): 100 GP
Castleview (30/23): 10 GP

Direction Sense:
The purple gem in the adventure screen shows which direction you are facing, as well as an arrow in the automap.
Druids start with it.
Dwarf Mine 5 (8/5): free (search skeleton)

Spot Secret Doors:
Shows walls you can bash in (a gargoyle in the adventuring screen will wave its arm).
Dwarves start with it.
Darkside (E4 5/12): 500 GP

Danger Sense:
The bat at the top of the adventuring screen will open its muoth when monsters are nearby.
Gnomes start with it.
Dwarf Mine 3 (5/11): free (search skeleton)
Castle Kalindra Lvl 1 (8/8): free

Arms Master:
Increases your chance to hit during combat.
Knights start with it.
Rivercity (30/3): 300 GP
Castle Kalindra Lvl 1 (8/6): free

Crusader:
Allows entrance to certain areas of the game. All party members must have it.
Paladins start with it.
Coudside (F4 9/3): free (after finding Falista's Alacorn)

Navigator:
You can travel through the desert without getting lost. 1 member must have it.
Rivercity (22/30): 2.000 GP
Sandcaster (19/5): 10.000 GP

Linguist:
Allows you to read cryptic runes.
Castle Burlock Lvl 1 (6/5): free
Sandcaster (19/27): 25.000 GP

Merchant:
Get better prizes in shops.
Sandcaster (19/27): 5.000 GP

Body Builder:
You will get more HP per level.
Rivercity (30/1): 1.000 GP

Prayermaster:
Cleric spell casters will get more SP per level.
Cloudside (A2 6/13)

Prestidigitator:
Sorcerer spell casters will get more SP per level.
Tower of High Magic Lvl 4 (7/8): free
Sandcaster (19/1): 1.000 gems

In Shangri-La you can learn all skills for 100.000 GP!