العلاقة بيني وبين صاحبي كويسة.

A natural Egyptian pronunciation is:

el-ʿalāʔa mabēni w mabēn ṣāḥbi kwayyisa

You may also hear something closer to:

il-ʿalāʔa bini w bin ṣāḥbi kwayyisa

A few notes:

• الـ is usually pronounced el- / il- in Egyptian.
• علاقة contains ع, which is a deep throat sound not found in English.
• كويسة is often pronounced kwayyisa / kweyyisa depending on speaker and region.

Word by word:

• العلاقة = the relationship
• بيني = between me
• وبين = and between
• صاحبي = my friend / my buddy
• كويسة = good

So the structure is basically:

the relationship between me and my friend is good

Even though there is no written word for is, Arabic often leaves it out in present-tense sentences like this.

In Arabic, especially in the present tense, the verb to be is usually omitted.

So instead of saying something like The relationship is good, Arabic simply says:

العلاقة ... كويسة literally: the relationship ... good

This is completely normal.
If you wanted past tense, then a form of كان might appear, for example:

العلاقة بيني وبين صاحبي كانت كويسة = The relationship between me and my friend was good

Because Arabic often repeats between with both sides of the relationship.

So:

• بيني وبين صاحبي = between me and my friend

This repeated pattern is very common and natural in Arabic.

Compare:

• بين أحمد ومحمد = between Ahmed and Mohamed
• بيني وبينك = between me and you
• بينها وبين أختها = between her and her sister

In English, we usually say between me and my friend with only one between, but in Arabic repeating it is standard.

Because بيني has a pronoun attached to it.

• بين = between
• بيني = between me
• بينك = between you
• بينه = between him
• بينها = between her

So:

• بيني = between me
• وبين صاحبي = and between my friend

The second part uses a noun phrase (my friend) rather than a suffix pronoun, so it stays as بين + noun.

In Egyptian Arabic, صاحبي is a very common everyday way to say my friend, especially in speech.

• صاحبي sounds natural and colloquial.
• صديقي is more formal or more associated with Modern Standard Arabic.

So in everyday Egyptian conversation, صاحبي is often the more natural choice.

Depending on context, صاحبي can also feel a bit like:

• my buddy
• my pal
• my close friend

كويسة means good / okay / fine in Egyptian Arabic.

It is a very common colloquial adjective.
By contrast, جيدة is more formal and more typical of Modern Standard Arabic.

So:

• كويسة = natural Egyptian speech
• جيدة = more formal / MSA

In daily Egyptian Arabic, people often prefer كويس / كويسة / كويسين.

Examples:

• أنا كويس = I’m good said by a male
• أنا كويسة = I’m good said by a female
• الحاجة دي كويسة = This thing is good

Because العلاقة is a feminine noun in Arabic.

Adjectives must agree with the noun they describe, so:

• علاقة كويسة = a good relationship
• not علاقة كويس

Compare:

• الوضع كويس = the situation is good
  because الوضع is masculine
• العلاقة كويسة = the relationship is good
  because العلاقة is feminine

So the ending on كويسة matches the gender of العلاقة.

Yes, it is natural and understandable Egyptian Arabic.

Egyptians might also say similar things such as:

• علاقتي بصاحبي كويسة = My relationship with my friend is good
• أنا وصاحبي علاقتنا كويسة = Me and my friend, our relationship is good
• الدنيا بيني وبين صاحبي كويسة = Things between me and my friend are good

So your sentence is perfectly normal, but there are several natural ways to express the same idea.

الـ is the Arabic definite article, meaning the.

So:

• علاقة = a relationship / relationship
• العلاقة = the relationship

In this sentence, Arabic uses the relationship because it is talking about the specific relationship between two people.

This is very natural in Arabic. English might also say the relationship, so the two languages match here pretty well.

Yes, many speakers would understand it, and it can sound natural in some contexts.

• علاقتي مع صاحبي كويسة = My relationship with my friend is good

However, علاقتي بصاحبي or العلاقة بيني وبين صاحبي often sounds more idiomatic when talking about the relationship connecting two people.

So:

• العلاقة بيني وبين صاحبي كويسة = very clear and natural
• علاقتي بصاحبي كويسة = also very natural
• علاقتي مع صاحبي كويسة = understandable and used, though sometimes بـ or بين may sound more native depending on speaker and context

Usually, yes. صاحبي literally means my male friend / my buddy.

If you mean my female friend, you would usually say:

• صاحبتي

So:

• صاحبي = my male friend
• صاحبتي = my female friend

That means the sentence as written most naturally refers to a male friend.

Yes. كويسة is flexible.

Depending on tone and context, it can mean:

• good
• fine
• okay
• all right

So the sentence may suggest anything from:

• the relationship is genuinely good to
• the relationship is okay / not bad

Tone of voice often helps show exactly how positive the speaker means it to be.