“a righteous person.” The Greek word dikaios (#1342 δίκαιος) is an adjective, and in this case is a substantive, an adjective used as a noun, “a righteous” referring to a “righteous person.” Using the substantive instead of just supplying the noun and saying “righteous person,” places the emphasis on “righteous.” If we want the reward of the righteous, we must receive “the righteous.”
“reward.” The Greek is misthos (#3408 μισθός), and it refers to a payment made for work done; wages. As “wages” or “payment,” it can refer to either a reward (cp. Matt. 5:12; 10:41; Luke 6:35; 1 Cor. 3:14) or a punishment (2 Pet. 2:13), depending on the context and what kind of payment is due. In the future Millennial Kingdom, when Jesus Christ rules as king on the earth, people will be repaid for what they have done (see commentary on 2 Cor. 5:10, “good or worthless”). Some people might think they have done very little to support God’s work, but if anyone has helped accomplish God’s work on earth, he will be amply rewarded.