by his, her, your (formal), or their side
Ella estuvo a su lado siempre que él tuvo problemas.
She was always by his side whenever he had problems.
Se quedará a su lado y poco a poco hará que se enamore de mí.
He will stay by her side and little by little make her fall in love with me.
— La doncella (2016)
Ella se arrodilló a su lado y le preguntó cómo se llamaba.
She knelt beside him and asked him what his name was.
Ella se sentó a su lado y escuchó en silencio.
She sat beside him and listened silently.
Cuando estás a su lado, te olvidas de todo.
When you're by her side, you forget everything.