Is it possible to see the Angel of Death in a dream or while awake, and does he inform of the time of death? What are his characteristics, and what should one who sees him do? Are there any previous instances where the Companions or others saw him?

We have no knowledge of the veracity of what the questioner mentioned. Seeing angels is possible, but for the Angel of Death to inform someone of their death date is incorrect, because only Allah knows the time of death, and it is among the unseen mentioned in His, the Almighty's, saying: "Indeed, Allah [alone] has knowledge of the Hour and sends down the rain and knows what is in the wombs. And no soul perceives what it will earn tomorrow, and no soul perceives in what land it will die. Indeed, Allah is Knowing and Acquainted." If a person suspects their death is near, they must repent, seek forgiveness, and persevere in righteous deeds.