tomodati ni ha himitu ni siteimasu.

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.

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.

It comes from 秘密にする.

• 秘密 = secret
• にする = to make into / to treat as / to keep as

So 秘密にする means:

• to keep something secret
• to make something a secret

Then 秘密にしています is the ている form:

• しています = is doing / has been doing / keeps doing

In this sentence, it means the speaker is in the ongoing state of keeping something secret.

Because keeping something secret is usually understood as an ongoing situation, not just a single momentary action.

Compare:

• 秘密にします = I will make it secret / I’ll keep it secret
• 秘密にしています = I am keeping it secret / I’ve been keeping it secret

So しています fits naturally when the secret is currently being maintained.

Correct — the thing being kept secret is omitted.

Japanese often leaves out information that is understood from context. So the sentence really means something like:

• I’m keeping it secret from my friend
• I’m keeping that a secret from my friend

The hidden part could be something like:

• そのことを = that matter
• これを = this
• 彼に会っていることを = the fact that I’m seeing him

A fuller sentence could be:

• そのことは友達には秘密にしています。
• I’m keeping that secret from my friend.

That is a very natural question.

に often does mean things like to, toward, or for, but its exact meaning depends on the verb or expression.

With 秘密にする, the person marked by に is often the person from whom the information is withheld.

So in this sentence:

• 友達には秘密にしています means
• I’m keeping it secret from my friend

This is one of those cases where you should learn the whole pattern:

• A に B を秘密にする
• to keep B secret from A

Because 秘密 here is not the direct object of する.

Instead, it is part of the pattern:

• Noun + にする

This pattern means:

• to make something into X
• to decide on X
• to leave something as X

Examples:

• 静かにする = make it quiet / be quiet
• きれいにする = make it clean
• 秘密にする = keep it secret

So 秘密をする would not be the normal expression here.

Yes, often at least a little.

友達には can suggest:

• as for my friend
• when it comes to my friend
• from my friend, specifically

This may imply contrast, such as:

• I told my family, but I’m keeping it secret from my friend
• Other people may know, but my friend doesn’t

However, the contrast does not have to be strong. Sometimes は is just used to gently highlight that part of the sentence.

Yes, absolutely.

• 友達には秘密にしています = slightly more contrastive or more focused on friend
• 友達に秘密にしています = more neutral

Both are grammatical. The version with は is often chosen when the speaker wants to emphasize from my friend in particular.

It could be either.

Japanese nouns usually do not show singular vs. plural unless context makes it clear.

So 友達 can mean:

• friend
• friends

This sentence could therefore mean:

• I’m keeping it secret from my friend or
• I’m keeping it secret from my friends

Context decides.

No, that would not be the normal interpretation.

The structure 秘密にしています means keeping something secret, not being a secret person or being hidden oneself.

So the natural reading is:

• I’m keeping it secret from my friend

not

• I myself am a secret to my friend

They are similar, but not identical.

• 秘密 = secret
• 内緒 = secret / confidential / just between us

In casual conversation, 内緒にしています can sound very natural too:

• 友達には内緒にしています。

That can feel a bit more conversational, like:

• I’m keeping it from my friend
• It’s a secret from my friend

秘密 is slightly broader and more standard as a general word for secret.

Yes, if you want to say exactly what is being kept secret.

For example:

• そのことを友達には秘密にしています。
• I’m keeping that matter secret from my friend.

Here:

• そのことを = the thing being hidden
• 友達には = from my friend
• 秘密にしています = am keeping secret

But in natural Japanese, leaving out the を phrase is very common when the listener already knows what the speaker is talking about.