You really need to supply a lot more information, like ....
What type of soil is it.
What the orientation is.
Is the depth of border 2' ? (from front to back).
What height the planting is to be.
How much water will it realistically get ?

All of these factors will result in specific types of plant because you can't just plant anything anywhere or for one of those reasons or another it's likely to fail.

Hi Suey and welcome to GOY this must be a sad thing for you. Its nice you are able to do this as usually there are a lot of restrictions around a grave.
As louise said little more information would help get you the right answers Perennials may not be the answer unless you can tend them quite often and I dont think they would define the 'C' unless planted on a very big scale.
To give an all year round appeal and to define the C shape, you could consider a low growing shrub like a boxus which needs a light trim only once or twice a year and could be kept to Ht 18"-2ft. You could then plant around this or just on one side some miniature bulbs these will soon be available to plant in the autumn for next spring, Snow drips, Crocus, Iris, Narcissus, Chinodoxia, Tulips which all come in a small variety to give a lovely long spring display together with primulas.
For the summer you could plant the biennials Sweet Williams and annuals such as Marigolds. Mesembryanthemum. Calendulas. and a late flowering short stemmed Aster.
I hope this helps.

Yes, welcome, Suey. I would go for alpines with miniature bulbs mixed in as Drc suggested. Alpines are small, there is interest for all seasons and they can be kept under control easily. And they don't need a great deal of attention.
I hope that whatever you decide on will give you pleasure for many years.