tomodachi ni wa himitsu ni shiteimasu.

I am keeping it secret from my friend.

Breakdown of tomodachi ni wa himitsu ni shiteimasu.

• は: topic particle
• 友とも達だち: friend
• する: to do
• に: target particle

Questions & Answers about tomodachi ni wa himitsu ni shiteimasu.

Why is は attached after に in 友達には?

This is a very common pattern: particle + は.

Here, に marks the person affected / the person something is directed toward, and は adds a sense of topic or contrast.

So 友達には suggests something like:

• as for my friend
• when it comes to my friend
• from my friend, at least

In this sentence, it often gives a slight contrastive nuance, as if the speaker is saying:

• I’m keeping it secret from my friend
• possibly implying but maybe not from someone else

Without は, 友達に秘密にしています is also possible and sounds more neutral.

Why are there two に particles in this sentence?

They are doing different jobs.

• 友達に: this に marks the person involved, here the person something is being kept secret from
• 秘密にする: this に is part of the grammar pattern Noun + にする, meaning to make something into X or to keep something as X

So:

• 友達に = with respect to a friend / from a friend
• 秘密にする = to make or keep something secret

They just happen to both be に, but they are not the same function.