Some people call this personality the Grumpy and the Viking personality. Their eyes look like slanted half circles that have hard edges. Their logo is a Furby dressed up like a Viking. How to Obtain: Overfeed Furby, Shake Furby, Turn Furby Upside Down. Because of how easy it is to get this personality, Furby can stay in this personality for a long time.
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''To change the personality you can:''
Play music to Furby, Sing for Furby, Use the app, Tilt furby from side to side, Pull its tail, Shake Furby, Tickle Furby, Rock Furby Slowly, Place another Furby beside it. There are 5 other personality's other than the evil one.
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There is a Default Personality also called clown, that you can't change back into it normally. You must factory reset by flipping your Furby upside down, while simultaneously holding the tongue sensor and the tail. After the eyes shut off, they will be reset back to this personality. But be careful, repeated resets might possibly corrupt the configuration memory.
There is also a Personality Changer App on Google Play. It used to be free but now its around 1$
Here is the link: https://play.google.com/store/apps/details?id=com.personalitychanger.full&hl=en_US&pli=1
Here is a video that may help: https://youtu.be/3NR2o9yrW80
Here is another video that may help: https://youtu.be/UteznTOUegE
You said that you have factory reset it before and it will become evil after a while. This leads me to believe that the Furby thinks it is being shaken, overfed or upside down.
-Make sure the tongue senor works, and is not jammed
-Lay the furby on a flat surface and see if it still reacts like it is being shaken, if it does the tilt sensor could be the problem.
I'm pretty sure the problem though is the orientation sensor. I knew someone with a similar problem. They opened him up, taking the front plate of the face. Then, found out that the sensor is a black box located on the left side of the furby. First, they tried to hit it a few times without any results. Then, they tried breaking into it by breaking part of the plastic box, and when it was broken, they turned him on, and now he works. I think it is a design flaw. The metallic ball was moving without any problem. To me it seems that the box is "too small" for the metallic ball itself... so it's always "closing" the circuit and every little movement is basically registered as a shake.
You can always contact me at Oliviarepairs@gmail.com for further advice.
I hope this helps, Thanks!