So I just started a new campaign with a few folks (If you are my few folks leave! I’m going to talk bad about you.). I created a magic item kinda spur of the moment for a quiet player to give me more DM/Player interaction with said player to bring her more into the game.

Her circumstances: Teifling rogue, chaotic neutral, literally fell out of the sky into a lake. Doesn’t remember anything prior.

The magic Item: NikNak (name derived from the names of the 2 skulls) - A pair of small skulls. An imp skull and a owl arcon skull. Sentient. Think of them as a devil on one shoulder and an angel on the other. They can speak out loud, but usually speak in the owners head. This telepathy seems to only work one way. The owner has to speak to the skulls, often making the owner seem crazy. The skulls may just be messing with her though.

Nikari - Imp skull abilities: Knows language abyssal/common. Grants owner +1 to listen checks.

Tanak - Owl arcon skull: Knows language celestial/common. Grants owner +1 to spot checks.

As a pair, grants +2 knowledge planes, and you can reroll a knowledge planes check if you succeed in a diplomacy check (dc 15 for easy questions, 20 for moderate, 25 for obscure, and 30 for secret) with one of the skulls. If you ask one skull the other won’t help you because it’s offended. When attempting diplomacy checks the player suffers a -2 penalty if they don’t refer to them using the skulls full name.

I want to have the items get more powerful as she does, and need to give them personalities. One kicker is Nik and Nak know where she is from, and won’t tell her.

I started the group at 3rd lvl. 3.5 Forgotten Realms by the way. I have a Monk with mad tumbles, a Ranger with a bow (duh), and a teifling rogue. No one picked a healer.

So, I would like help with:
1. Personalities for both skulls and arguments they could have with each other, funny stuff a plus.
2. Why won’t they tell the girl where she is from and about her past. What is her past?
3. What kind of bonuses should the skulls get as she gets higher in level.

I think that you need to read Unearthed Arcana's "Item Familiar" section. You might not require the player to actually spend the feat, thus, avoiding the XP bonuses s/he'd might get.

Have you ever played Black and White? The PC game where you're a god and you have 2 little outsiders offering advice? Sometimes they even argue among themselves and it's funny because they talk to each other when it's a topic both are interested in (saving a character that wronged you, for an example. The good one sees an opportunity of personal growth and forgiveness, the evil knows a perfect revenge chance when he sees it). I'd suggest, also, that to make things more fun, don't make the good one mechanical. Make it like a paladin (lawful good, tons of charisma), so the character would actually hear both parts before taking a decision.

<_< for the evil one, you could also see Looking For Group on youtube, the song performed by Richard. Evil and funny.
"You've nowhere to hide! nowhere to run!
your village will BURN like the heart of the suuuun..."

I will have to dig through my books for Unearthed Arcana and check it out. I still need to come up for a backstory and add in how she got NikNak.

I will have to dig through my books for Unearthed Arcana and check it out. I still need to come up for a backstory and add in how she got NikNak.

In mechanus, in a small court, an item was designed to know what would a counterpart of a certain alignment would do. They got 2 different skulls of very different beings that were somewhat akin to a jury that would state their opinion. Since they saw that adding evil and good doesn't necessarily end up being neutral, the item was locked away and forgotten about (but not mislabeled nor without records). That could be the origin. How it came to be a part of a tiefling's inventory can be a fun story or an incredibly simple one that could leave the PC unsuspecting about its true origins and purpose, not to mention rarity.