After doing some investigation, it seems that there are two Unicode characters which are made of a circle inside another circle:
- ◎ U+25CE BULLSEYE
- ⭗ U+2B57 HEAVY CIRCLE WITH CIRCLE INSIDE
In this Japanese context, only the first one ◎ appears to be appropriate (Wiktionary), since it is known as 二重丸【にじゅうまる】 and is defined as 「とても良いことを示す記号。」, which suggests that it is indeed a stronger (double) variation of the more commonly used symbol ○ 丸【まる】「良いことを示す記号。」(Wiktionary).
So, 性格も◎ can probably be understood as something like "very good character too"...