We never use the perfect tense of the following verbs: modal verbs (e.g. konnte(can)), haben(to have), sein(to be), and wissen(to know). For example, these sentences still sound casual:
Ich konnte dich sehen.
I could see you.
Das wusste ich nichts.
I didn’t know that.
Additionally, in a casual context, you can use the verbs kommen(to come), gehen(to go), or finden(to have an opinion), in the perfect tense or the simple past.