In rabbinic literature, the Satan is depicted as having a personality, and yetzer hara is equated to Satan.
Obviously the Satan as a character is found in the beginning of Job:
1:6 וַיְהִי הַיּוֹם--וַיָּבֹאוּ בְּנֵי הָאֱלֹהִים, לְהִתְיַצֵּב עַל-יְהוָה; וַיָּבוֹא גַם-הַשָּׂטָן, בְּתוֹכָם.
Now it fell upon a day, that the sons of God came to present themselves before the LORD, and Satan came also among them.
Even in rabbinic literature, he gets a personality, even if it might be interpreted metaphorically by those more rationalistically inclined. For example, see Sukkah 52b that says the Satan will testify against man in the next world, according to Rabbi Yonatan.
However, rabbinic thought sees the Yetzer Hara and Satan as the same (at the very least some see it that way), such as found in Bava Batra 16a:
אמר ר"ל הוא שטן הוא יצר הרע הוא מלאך המות הוא שטן דכתיב ויצא השטן מאת פני ה' הוא יצר הרע כתיב התם רק רע כל היום וכתיב הכא (רק את נפשו שמור) [רק אליו אל תשלח ידך] הוא מלאך המות דכתיב (רק) [אך] את נפשו שמור אלמא בדידיה קיימא א]
Resh Lakish said: Satan, the evil prompter, and the Angel of Death are all one. He is called Satan, as it is written, And Satan went forth from the presence of the Lord. He is called the evil prompter: [we know this because] it is written in another place, [Every imagination of the thoughts of his heart] was only evil continually, and it is written here [in connection with Satan] 'Only upon himself put not forth thine hand. The same is also the Angel of Death, since it says, Only spare his life, which shows that Job's life belonged to him.
Perhaps the language of an actual being (indicated by the definite article) remained in the Hebrew, as a relic of a time in rabbinic history when it was an external character. However, the Yetzer Tov was never really an actual being, and never had to keep the definite article.
Also, I don't have a source for this, but it makes sense that it's simply hard to pronounce two reshes in a row, so the definite article gets put before it to separate them. Certainly, reshes get played with a lot in grammar when they are pitted against other reshes (people who know Hebrew dikduk will get this).