1. Yuyuko's fan
Gameplay-wise, the fan gives her a huge extra hitbox that takes 1/3 damage compared to directly attacking Yuyuko, so you can keep damaging Yuyuko, even when slowly moving through her dense danmaku.
The symbology of this fan deserves another post.
2. Ran's Buddha
Ran's symbol looks like a typical small Buddhist portrait or icon, probably the kind drawn or carved onto a talisman.
3. Yukari's eight-pointed "snowflake"
It's the same symbol that rotates on PC when you focus. It's also the background of ESoD difficulty selection screen. Another special appearance of the symbol is in IN. When a bosss charges up for a special attack in IN, the special effect is many "snowflake" symbols gathering on the character.
SaBND also referenced Yukari's symbol, when she was getting into a battle position to test the fairies' strength.
My personal theory is, this is the symbol of spell card rules.
4. Byakuren's butterfly
It's a butterfly, since her spell cards Magic "Bewitching Fragrance of Makai Butterfly" / Magic "Magic Butterfly" referenced it. This one seems to have no meaning other than referencing Shinki... It will be a Taoist reference if appearing on a Taoist character.
5. Miko's halo
Contrary to popular belief in the Western fandom, this isn't a Taoist butterfly reference. Instead this is a reference to the fiery halo behind Buddhist statues. Prince Shoutoku was thought to be an avatar of the bodhisattva Kannon, and this is referenced by Miko's spell cards Light Sign "Halo of Guse Kannon / Light Sign "Guse Flash" ("guse" is Kannon's title, "world-saving").
Before delving into Yuyuko's fan, I want to post about the various boss charge-up and explosion effects since PCB, simply because I had recently thought about it. Skip to the next post if you are uninterested!
In PCB, the effect is snowflakes for Stage 1-4 (so even Lily White exploded into snowflakes), and sakura petals for Stage 5-Phantasm. Aside from relation to story, this also effectively makes PCB one half winter and one half spring in terms of stage numbers.
In IN, all bosses explode into maple leaves, the symbol of autumn. For charge-up, most bosses use the eight-pointed symbol, except for Wriggle who blinks.
In StB, the bosses' explosion effect is plain and quick.
In MoF, all bosses charge-up and explode with maple leaves. However, ZUN somehow didn't do anything to change this effect for the subsequent games set in different seasons, until TD, where it's at last replaced by sakura petals, in keeping with the spring season.
Since DDC is set in autumn, the effect is maple leaves again.
In summary, the symbols for each season (also see the "now loading" effects):
Spring - sakura
Summer - no idea, the summer games are ESoD and fighting games
Autumn - maple leaves
Winter - snowflakes