We claim that all of the Companions are upright. By “Companion” we mean a Muslim who was in the presence of the Prophet ﷺ during his mission. By “upright” we mean that they intend good for the religion, such that we can be certain that a Companion would never deliberately fabricate a lie against the Prophet ﷺ. Consequently, we also claim that no matter the sins a Companion commits, whatever narration he reports from the Prophet ﷺ should never be dismissed as a fabrication.
To the above, the Twelver might ask: “this is special pleading for the Companions. You do not accept narrations from those who commit major sins, so why make an exception for the Companions specifically?”
The response to the above is two-fold. First by clarifying why narrations are typically not accepted from those who commit major sins. Second by using this clarification to justify the exclusion of the Companions from this principle.
As for why narrations are typically not accepted from those who commit major sins: this is because piety is a sign that the pious is God-fearing, and those who are God-fearing are of pure intentions, and will therefore not deliberately lie about the Messenger of Allah ﷺ. Thus, those who abstain from major sins are more likely to accurately relay what they have received from the Prophet ﷺ. Inversely, those who commit major sins are less likely to accurately relay narrations.
Notice that all of the above is circumstantial. We are judging the intentions of people based on what we see those people do. We don’t actually know what their true intentions are. After all, it is not impossible for those who commit major sins to have pure intentions and accurately relay what they received from the Prophet ﷺ. It is also not impossible for those who (apparently) abstain from major sins, to fabricate lies against the Prophet ﷺ. Indeed, this is one of the reasons why singular narrations are indecisively reliable (Dhani Al-Thubut), as opposed to being decisively reliable (Qat’i Al-Thubut).
Given the probabilistic nature of the above assessment, we deduce the following: if we can somehow know the true intentions of a person, then it doesn’t matter whether or not this person commits any sins. We do not need to rely on their actions as probabilistic evidence for or against their integrity, since we already know what this person’s intentions are.
It should now be clear why the Companions are taken as an exception to the above principle. It is because we do know that their intentions are pure, so we do not need to depend on their actions to prove it. This is based upon what Allah ﷻ told us about them in His book:
مُّحَمَّدٌ رَّسُولُ اللَّهِ ۚ وَالَّذِينَ مَعَهُ أَشِدَّاءُ عَلَى الْكُفَّارِ رُحَمَاءُ بَيْنَهُمْ ۖ تَرَاهُمْ رُكَّعًا سُجَّدًا يَبْتَغُونَ فَضْلًا مِّنَ اللَّهِ وَرِضْوَانًا
Muhammed is the Messenger of Allah, and those who are with him are firm against the disbelievers, merciful towards one another. You see them bowing and prostrating, seeking grace from Allah and His pleasure.
The verse defines exactly those who it praises. The verse praises those Muslims (“You see them bowing and prostrating”), who were in the presence of the Prophet ﷺ (“Muhammed is the Messenger of Allah, and those who are with him”). This definition perfectly matches our definition for “Companion”.
Next, this verse tells us about the intentions of the Companions. It tells us that they intend to please Allah ﷻ (“seeking grace from Allah and His pleasure”). And it is impossible for the one who intends to please Allah ﷻ, to then fabricate lies about the Prophet ﷺ.
Quran 48:29 is as such, conclusive proof for the integrity of all Companions. No amount of circumstantial evidence can change this fact.
To the above the Twelver might respond: “this is ultimately a circular argument. The authenticity of the Quran itself depends on the integrity of the Companions. So how can you use it to prove that they are upright?”
We respond: the Quran is mass transmitted, and mass transmitted reports do not depend on the integrity of the reporters. This is because the many independent narrators relaying a mass transmitted report, corroborate each other’s testimony. So we can be certain that they didn’t fabricate the report. It is for this reason that even that which has been mass transmitted from disbelievers, is accepted as a source of knowledge.
Finally someone might ask: “does this mean that it is impossible for a Companion to make a mistake when narrating from the Prophet ﷺ?”
The response is no. Our claim is that the Companions will not deliberately lie about the Prophet ﷺ, but that doesn’t make this Companion immune from committing honest mistakes.
 Quran 48:29
 For example: it is possible for a Companion to inaccurately report from the Prophet ﷺ due to memory lapses. It is also possible for a Companion to have misheard something from the Prophet, and for this Companion to then relay what he misheard (as opposed to what was actually said).
The only way to avoid all of the above, is through mass transmission.