So here are his stats if you look his stats say his speed is good so Timid may be a good choice as his SDef and Def is low. He can then rely on his speed.
As for Modest, his attack is very low so modest is an option but he becomes very frail with a middling speed and def and SDef to protect himself.
I'd say Timid because his attack is already so low and he needs a stat to rely on (in this case speed). Your Rotom has now got a definite nature!
For other natures, click right here.
Hope I helped!!! :D