Please note that **Upa** has more than one meaning. In this context, it should not be considered as Junior or Deputy or Assistant. So helper is not right word but friend if it is referring Vamana or other incarnation of Vishnu. But assistant is right word if name of one of Aditi's sons was Vishnu. Or if we consider that Vamana was younger child of Aditi so can be addressed with **Upa** when we talk in respect to Aditi's elder sons.

Adityas means

> sons of Aditi

If Vishnu is referred as Aditya that means 

> either **a son of Aditi whose name is Vishnu** 

> or Vishnu's incarnation **Vamana** (took birth as a son of Aditi).

> But not the Lord Vishnu from the Trinity gods directly.

**Why Lord Vishnu from Trinity can't be Aditya?**
> Aditi was Daksha's daughter and as I mentioned Adityas means sons of Aditi. But Trinity gods were already there before the birth of Aditi.

If Aditi named his one son after the name of Lord Vishnu then this Vishnu is not same as the Lord Vishnu from Trinity. Or if from context, it is clear that it is referring Vamana incarnation then these is no confusion at all.

And we all know that **Vamana incarnation** just happened to help Indra because Bali was about to snatch Indra's throne.

**Mohini incarnation** also helped Devas.

**Krishna incarnation** too fought for Devas and killed Narkasura. Even though there was some initial conflict and Krishna encouraged people to worship nature (Govardhan) and not Indra. But Indra and Krishna both were good friends. From Indra, Krishna brought Parijat tree to his wife.

Indra sent his chariot to **Rama incarnation** in the battle with Ravana.

In all incarnation Vishnu and Indra seems to be friend. Which purana says that Indra and Vishnu were or are enemies?

**Update:** As per Rakesh Joshi's comment

Friend doesn't mean they can not be devotee to each other. For instance,

> Lord Shi Rama (an incarnation of Lord Vishnu) is a great devotee to Lord Shiva.

> Lord Hanuman (11th Rudra, closely related to Lord Shiva) is a great devotee to Lord Shi Rama.

Does that make Shiva inferior to Vishnu or vice versa? Aren't they good friends and should be considered equal?