Mi amiga senderista siempre lleva una cantimplora llena de agua.

Because mi without an accent is the possessive adjective meaning my.

• mi amiga = my friend
• mí with an accent is a pronoun meaning me after a preposition:
  • para mí = for me
  • sin mí = without me

So here, mi amiga is correct because it means my friend.

Amiga is the feminine form of amigo.

Since the sentence is talking about a female friend, Spanish uses:

• amigo = male friend
• amiga = female friend

This also affects other words that describe her, such as llena, which is feminine too.

Senderista means hiker or person who goes hiking.

In Spain, senderismo means hiking, especially walking on trails or in the countryside. So:

• un senderista = a male hiker
• una senderista = a female hiker

It is a very natural word in Spain Spanish.

Some Spanish nouns referring to people have the same form for masculine and feminine, and senderista is one of them.

The gender is shown by:

• the article: el senderista / la senderista
• or other words around it: mi amiga senderista

So senderista works for both men and women.

It can be understood as either, and that is why it may feel a little tricky.

In mi amiga senderista, it behaves like a descriptive noun in apposition or like a noun used adjectivally:

• mi amiga = my friend
• senderista = who is a hiker / hiking friend

A natural way to understand it is:

• my friend, who is a hiker
• my hiking friend

Spanish often places this kind of descriptive word after the noun.

Because siempre is an adverb meaning always, and in Spanish it commonly goes before the conjugated verb:

• Siempre lleva... = She always carries...

That is one of the most normal positions. You can also sometimes move adverbs for emphasis, but siempre lleva is the standard, natural order here.

Lleva is the third person singular of llevar in the present tense.

The subject is mi amiga senderista, which is she, so you use:

• yo llevo = I carry
• tú llevas = you carry
• ella lleva = she carries

So mi amiga senderista siempre lleva... means my hiking friend always carries...

Usually llevar is the right verb here because it means to carry / to take with you.

• llevar = to carry, wear, take along
• traer = to bring

In this sentence, the idea is that she has it with her regularly, so lleva is the natural choice.

Traer would usually suggest bringing something toward the speaker or another reference point, which is not the main idea here.

Because cantimplora is a countable singular noun, so Spanish normally uses an article with it:

• una cantimplora = a water bottle / canteen

Spanish often uses articles in places where English sometimes omits them, but here both languages use one naturally:

• a canteen
• una cantimplora

A cantimplora is a canteen or water flask, especially one used for hiking, camping, or outdoor activities.

It is not exactly the same as any bottle in general:

• botella = bottle
• cantimplora = canteen / hiking flask

So in this sentence, cantimplora fits the hiking context very well.

Because llena agrees with cantimplora, which is a feminine singular noun.

• una cantimplora llena = a full canteen
• un vaso lleno = a full glass

In Spanish, adjectives usually agree in gender and number with the noun they describe.

So:

• cantimplora → feminine singular
• llena → feminine singular

Because the adjective lleno/a is normally followed by de when you say what something is full of.

• lleno de agua = full of water
• llena de libros = full of books
• lleno de gente = full of people

So de is required here.

Because the sentence is talking about the contents of the canteen in a general sense: it is full of water.

With lleno/a de, Spanish usually does not use the article when speaking generally about a substance or material:

• llena de agua
• lleno de vino
• llena de arena

You would use del agua only in more specific contexts, for example if you meant full of the water from a particular place or source. That is not the idea here.

It could, but it is not necessary.

• lleva una cantimplora... already means she carries a canteen
• lleva consigo una cantimplora... adds the idea of with her

So consigo would make it a bit more explicit, but the original sentence is perfectly natural and simpler.

Yes. Both are good ways to understand it.

Depending on context, it can suggest:

• my friend who is a hiker
• my friend, the hiker
• my hiking friend

Spanish often allows this compact kind of description without needing a longer relative clause like que es senderista.

In most of Spain today, ll is pronounced the same as y. This is called yeísmo.

So llena is usually pronounced roughly like:

• YE-na

A more precise approximation is /ˈʝena/ in many accents.

In some areas, people still distinguish ll from y, but for most learners of Spain Spanish, pronouncing it like y is perfectly normal.

Mi amiga senderista is the natural order here.

In Spanish, descriptive words like this usually come after the noun:

• amiga senderista

Saying mi senderista amiga would sound unusual in normal speech and could feel literary or unnatural. For a learner, it is best to keep senderista after amiga here.