Radak #1: It is To'ar (kind of person); he goes in the pure path - "Holech b'Derech Tamim Hu Yesharseni" (101:6).

Radak #2: It is a noun, like "Im be'Emes uv'Samim" (Shoftim 9:16). He goes in purity - "va'Ani b'Sumi Elech" (26:11). The meaning is the same (whether it is To'ar or a noun - refer to 15:2:1:1). It is one who engages in matters of this world simply, and does not delve into worldly strategies.

Malbim: In Mitzvos Bein Adam l'Makom he goes purely. I.e. all his deeds are solely l'Shem Hashem, without ulterior motives.

Makos 24a: Aba Chilkiya illustrates this. ¹

Radak: All his deeds are with Tzedek (righteousness).

Malbim: In Mitzvos Bein Adam l'Chavero (he is righteous).

Rashi (3): The good that he says, it is true in his heart. His mouth and heart are the same.

Radak: (a) He speaks Emes, and never Sheker. It is in his mouth and in his heart; there is no difference between them. Just like his mouth's words are Emes, also his heart's thoughts ¹ . (b) He fulfills his heart's intent to do good regarding buying and selling. It goes without saying that he fulfills his word; even what he intends to fulfill, he makes his intent Emes. (c) Elokim's existence and unity, which is Emes, he speaks with his mouth and thinks it in his heart to understand it via proofs, like his Chachmah instructs him.

Malbim: His Emunah is not merely external on his lips; his heart agrees to what his lips say.

Radak: There are three aspects to man, both regarding [Mitzvos] Aseh and Lo Sa'aseh - deed, speech and intent. "Holech Tamim u'Fo'el Tzedek" refers to deed; "v'Dover Emes bi'Lvavo" applies to speech and intent.