Formal:
* Indifferent: Lacking interest or concern.
* Apathetic: Showing or feeling no interest, enthusiasm, or concern.
* Unconcerned: Not worried or anxious.
* Detached: Emotionally uninvolved.
* Impassive: Not showing any emotion.
Informal:
* Casual: Relaxed and informal.
* Easygoing: Relaxed and tolerant.
* Carefree: Free from worry.
* Unfazed: Not disturbed or upset.
* Cool: Calm and collected.
* Chill: Relaxed and calm.
More specific:
* Insouciant: Carefree and lighthearted.
* Debonair: Confident and charming.
* Blase: World-weary and indifferent.
* Flippant: Lacking seriousness or respect.
The best synonym to use will depend on the specific context and the nuances of meaning you want to convey.