Party Role
Ridley is a melee skirmisher, so belongs darting in and out of the front lines. She is not particularly resilient, so she needs to be careful about her placement so she doesn't draw too much attention/damage. For the most part she shouldn't be fighting alone unless her combatant is fairly week. With her Dirty Fighter trait she deals one extra damage when flanking, which may not sound like a lot, but the key to Ridley's combat ability is lots of small bonuses adding up.
When the fighting is serious and she needs to start investing resources Ridley's go to power is Archaeologist's luck. She has seven uses of the power (she gains more as her Charisma increases) per day, but can spread them out throughout the day as she needs. The basic ability gives her a +1 to hit, damage, skill checks, and saving throws until next level when it becomes +2...but in combination with her Fate's Favored trait she gains +2 right now to all that, and will gain +3 next level.
Ridley is also a fount of knowledge, which will help in and out of combat. Chances are very good she will have the appropriate Knowledge skill to identify the various critters she comes across, which will help her with information like "undead are immune to mind-affect effects, like charms or compulsions" or "goblins are terrified of horses and dogs."
Out of combat her Knowledge skills are just as, if not more useful. She knows the region and its history fairly well, which will come in handy as the campaign unfolds.
Her ranks in Use Magic Device deserve special note. This is one of the more powerful skills in the game, letting Ridley read Scrolls and use Wands she otherwise wouldn't be able to do anything with. A character with UMD can also use items that are restricted to specific classes or races or have other prerequisites. Find a sword that has extra magic powers if wielded by an elf? Not a problem, you can trick the sword into thinking you're an elf.
Spellcasting
Right now Ridley has a few spells that give penalties to her opponents, with Grease and Confusion, but most of her casting is focused on making her or other members of her party better.
Things to Remember
With Uncanny Dodge Ridley doesn't get flat footed. The only thing that keeps you from gaining your Dex bonus to AC is being immobilized.
Halflings get bonuses to saves vs Fear. Make sure if you are asked to make a Will save you're getting this bonus when it applies.
Being something of a trap expert Ridley has Trap Sense, so if things go sideways and a trap is sprung she gets a bonus to her AC or Reflex against the trap's nastiness.
Luck bonuses are important to keep an eye out for. Ridley's Fate's Favored trait makes every luck bonus she get act as 1 higher.
Being an expert dungeon delver and crypt breaker, Ridley gets bonuses to a couple different things when in underground settings. Look over Terrain Mastery above, and make sure you're not forgetting to add those bonuses when they apply.
