I am quite a hard worker and I can confidently say that I am quite a reliable person.

confidently 是形容词 confident 的副词形式，由 confident（自信的）加上副词后缀 -ly 构成。confidently 表示"自信地、有把握地"，用来修饰动词 say，说明说话时的态度是充满自信的。