Someone who is observant 【pays a lot of attention to things and notices more about them than most people do】.
后面括号中的话是对前面内容中那个词的修饰呢?是修饰observant?someone?还是前面整句话呢?为什么没有连接词?
who is observant 是定语从句,先行词是Someone。
Someone ... pays a lot of attention to things and notices more about them than most people do 是主句。