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As described in the link manual, in One to One relationships you must create a function that returns an object from namespace \Illuminate\Database\Eloquent\Relations\ , this namespace has many types of relationships.

Assembling the functions where the person belongs to a user, therefore in the Person model:

public function user() {
    return $this->belongsTo(App\User::class);
}

And a user has one person, this way in the User model:

public function pessoa() {
     return $this->hasOne(App\Pessoa::class);
}

The relation of type belongsTo implies that the foreign key exists in this table, so according to its description, in this case we read that Person belongs to a User.

And because Eloquent tries to resolve the foreign key name according to the name of the related classes, it would be interesting to use the name user_id for the field in the person table. If you want to use user yourself, you must pass the custom key name to the functions:

Model Person :

public function user() {
    return $this->belongsTo(App\User::class, 'user');
}

Model User :

public function pessoa() {
     return $this->hasOne(App\Pessoa::class, 'user');
}

Now to retrieve the person or user related to a record, simply call the function name. For example:

$p = Pessoa::find(xx);
return $p->user->email;

$u = User::find(yy);
return $u->pessoa->sobrenome;

If each person corresponds to a user then the laravel relationship is:

In the model Person:

public function user() {
        return $this->belongsTo('App\User');
    }

In the User model:

 public function pessoa() {
            return $this->belongsTo('App\Pessoa');
        }