"It depends on what your definition of 'is' is" -Bill Clinton
He got made fun of for saying that, but when you think about "to be" really means so many things in English. The sentence "I am" is so powerful because of it's ambiguity, and multiple meanings.
Reisu doesn't have such an over arching concept as this verb does in Engish. It's more broken down.
Kuala - To be the same as in identity.
Al kuala tivu.
Al is the suit (of armor).
Luta - To belong to a specified class or group
Eisa luta emodo.
We are people.
Maino - To have or show a specified quality or characteristic
Denu maino gura.
The baby is hungry.
In many cases 'hola' for feel can also be used. This is mostly for first and second person subjects. For example this same sentence in first person: 'Ei hola gura'. I am hungry. (literally: feel hungry). But "Denu hola gura.", while comprehensible sounds awkward.
Reiki - To consist of, or to be made of
Laza reiki aa xusu.
The ocean is all water.
Lamu - To be like, or have specified significance.
E lamu komonaxi.
He's (like) a lion.
Kutu - To exist
Ei geki, mo ei kutu.
I think, therefore I am
There are lots of other ways we use 'to be' of course, but we could just as well substitute other verbs even in English, so I won't cover that in this post, or it will become too long :)
One other fun thing with copula-type verbs in Reisu is they can be omitted if the contexts provides sufficient clues to render the copula obvious. In this case the object becomes a verb. For example:
Ei okila bofi doro fe mama eino. E larelaila, kata e xodila
I saw an old picture of my mom. She was beautiful, when she was young.
In this sentence it's obvious we are saying that the mother has beauty and has youth (maino). So instead of saying "E mainola larelai, kata e mainola xodi" we make larelai (beautiful) and xodi (young) into verbs. Both "E mainola larelai" and "E larelaila" are correct and appropriate in this context.