[@Omni7658]Reminds me of the anime movie Jin-roh. A brilliant but twisted take on the Red Riding Hood tale, like a true germanic/nordic folkstory going: "Going down the path of pins or the paths of needles..." Loved that line.

You could say you have more affinity for a particular type of magic, it might be determined by one's own inward desires and will.
Like with Arn it is all Elemental; which could be considered as wild forces, his need 'to gain control over certain aspects in his life' like the situation with his mother might have lead to him needing to unleash his own anger and need for control through magic. Or at least that's the impression I get from him. It could also be, why he isn't particularly emotive or tends to hide his emotions.

Whilst with Eilis it is quite the opposite, in her early years she wanted to help, to literally lend a hand. Hers were a creation of good intentions or humble intentions, however when they wanted her to 'harm people' with it she withdrew, seeing the negative side to her own powers. So whenever she uses her magic now especially with the binds she does so reluctantly and with a great deal of fear driving her.
Which is why when 'shit will hit the fan' it will be all the harder for her to stop, for fear is a powerful motivator.