For some background as to who Reuben is - as far as the Torah has told us - read Genesis 29:32.
Also, consider Genesis 29:28-32 and 33:1-3 and 35:16-21
(ה) אמר ר׳ שמואל בר נחמני אמר ר׳ יונתן כל האומר ראובן חטא אינו אלא טועה שנאמר (בראשית לה כב) ויהיו בני יעקב שנים עשר מלמד ששקולים כאחד אלא מה אני מקיים (שם) וישכב את בלהה פילגש אביו. מלמד שבלבל מצעו של אביו ומעלה עליו הכתוב כאילו שכב את בלהה. תני רשב״א אומר מוצל אותו צדיק מאותו עון ולא בא מעשה זה לידו אפשר עתיד זרעו לעמוד על הר עיבל ולומר (דברים כז כ) ארור שוכב עם אשת אביו ויבא חטא זה לידו אלא מה אני מקיים וישכב את בלהה פילגש אביה עלבון אמו תבע. אמר אם אחות אמי היתה צרה לאמי שפחת אחות אמי תהא צרה לאמי ובלבל את מצעה. אחרים אומרים ב׳ מצעות בלבל אחת של שכינה ואחת של אביו. והיינו דכתיב (שם מט ד) אז חללה יצועי עלה א״ת יצועי אלא יצועיי:
(5) R Samuel b. Nachmeini said in the name of R. Jonathan: "Whoever says that Reuben (the son of Jacob) sinned, errs, for it is said (Gen. 35, 22.) Now the sons of Jacob were twelve. It is intended to inform us that they were all equal [in righteousness]. How then shall we explain the first part of the above-mentioned passage? It is intended to teach that he (Reuben) deranged his father's bed, and the Scriptures charge him as if he had been lying with Bilhah." We are taught that R. Simon h. Elazar said: "That righteous one (Reuben) is cleared of that crime, that such an occurrence never happened to him, for how could it possibly be that a man whose descendants were to stand on Mt. Ebal and proclaim (Deu. 29, 20.) Cursed be he who lieth with his father's wife, would commit such a sin. But how then is the passage (Gen. 35, 22 ) And he lay with Bilhah, his father's concubine, to be explained? It is intended to inform us that he demanded redress for the humiliation inflicted upon his mother saying: 'When my mothers sister lived and proved a vexation to her, it was bearable; but that the servant of my mother's sister should be a vexation to my mother is unbearable!' Whereupon he went and deranged the bed of Bilhah." Others say he deranged two beds, that of the Schechina and that of his father, and this explains that which is written (Gen. 48, 4.) Unstable as water, thou shalt not have the excellence, because thou did go up to thy father's bed; then didst thou defile the Shechina of my couch. Do not read Yetzu'ey (my bed), but read Yetzuay (the beds).
(יט) וַיָּשָׁב רְאוּבֵן אֶל הַבּוֹר (בראשית לז, כט), וְהֵיכָן הָיָה, רַבִּי אֱלִיעֶזֶר וְרַבִּי יְהוֹשֻׁעַ, רַבִּי אֱלִיעֶזֶר אוֹמֵר בְּשַׂקּוֹ וּבְתַעֲנִיתוֹ, כְּשֶׁנִּפְנָה הָלַךְ וְהֵצִיץ לְאוֹתוֹ בּוֹר, הֲדָא הוּא דִכְתִיב: וַיָּשָׁב רְאוּבֵן אֶל הַבּוֹר, אָמַר לוֹ הַקָּדוֹשׁ בָּרוּךְ הוּא מֵעוֹלָם לֹא חָטָא אָדָם לְפָנַי וְעָשָׂה תְּשׁוּבָה, וְאַתָּה פָּתַחְתָּ בִּתְשׁוּבָה תְּחִלָּה, חַיֶּיךָ שֶׁבֶּן בִּנְךָ עוֹמֵד וּפוֹתֵחַ בִּתְשׁוּבָה תְּחִלָּה, וְאֵיזֶה זֶה הוֹשֵׁעַ, שֶׁנֶּאֱמַר (הושע יד, ב): שׁוּבָה יִשְׂרָאֵל עַד ה' אֱלֹהֶיךָ.
And Reuben returned to the pit (Gen 37:29). Where had he been? R. Eliezer and Rabbi Joshua reported that R. Eleazar said: He had been dressed in sackcloth and fasted; when he was finished he went and looked in the pit. Hence it is written, "And Reuben repented." R. Joshua said: All the affairs of the house were committed to him; but as soon as he became free, he went and looked in the pit. Thus it is written, "And Reuben returned to the pit, etc." Said the Holy One to him, [Reuben]: 'No man has before repented after sinning before Me, you are the first to do so. As you live, your descendants will stand forth and be the first to urge repentance.' To whom does this allude? To Hosea, who cried out, "Return, O Israel, unto the Eternal, your God (Hos 14:2).